Population aging refers to an increasing median age and a decreasing young age in the population. Generally, an aging society means people over 60 account for over 10% of total population or people over 65 make up over 7% of total population [4] . In order to cope with increasingly serious aging problem, many theories and policies has been presented over last 70 years. Successful aging, productive aging, healthy aging and active aging are the famous theories that cope with aging problem from positive angle. The concept of successful aging can be traced back to the 1950s, it refers to the process of developing and maintain the functional ability that enables well-being in old age [5] . Productive age was first presented by Robert in 1983, it means the participation of older people in various activities that devote to generate goods or services or develop the capacity to produce goods or service [6] . This notion mainly focuses on body health. In 1990, the World Health Organization stated a new notion which named healthy aging. This notion extends the scope of health to cover both body and mentality [7] . Table 1 displays the development from successful aging to health aging. In 1996, active aging was presented, it become a more comprehensive concept by refining the content of successful aging, productive aging and healthy aging. Health, participation and welfare are three key component of active aging.

Health means the old group maintains ability to keep health on body and mentality. Thus, they can constantly adapt to social change [8] . Participation means the old group joins the social development positively and promote the social development initiatively [9] . Welfare refers to the society is accountable to the old group for their wellbeing. To achieve their wellbeing, the society is supposed to construct a whole system which includes policy support, resource supply and monitor method [10] (Figure 1).

Activity aging revolutionizes the values on aging process. Over a long period, the old group has been considered as the burden of society because the more percentage of old people means the less population of labour force. However, activity aging transfers this negative perspective to positive view. In this theory, the old group is not burden but treasures, human society will benefit from their life experiences and wisdom. Those old people could use their own way to join and promote the social development, such as volunteering and reemployment [11] .

Moreover, active aging is a dynamic process which accompanies with the whole lifespan of human being. It means people should prepare active aging at an early age. Hence, the maintenance of health, the building of participation interest and the development of social welfare system are supposed to be with people in their whole lives [12] .

The embryo of positive psychology could be tracked back to Terman’s discussion and Jung’s study in the 1930s. In the 1960s Humanistic psychology generated the research on human potential, and this further promoted the development of positive psychology. However, due to the impact of World War II, the mainstream of psychological research transferred to pathological psychology and negative psychology. The main duty of psychology during that period was to heal the trauma of war [13] .

In the end of the 20^th century, peace and development become the main topic worldwide. In this background, the main duty of psychology changed, more psychological research attempted to achieve social sustainability and provide a better life for human being. Thus, many new tasks, such as virtue cultivation, character strength and potential achievement were presented in psychological field.

Positive psychology raised in this background, the main purpose of this discipline is to understand and stimulate human virtue. It advocates exploring human nature from a positive angle and obtaining positive power through the process of exploration [14] . The value of “positivity” is emphasized in the theory of positive psychology because it believes those positive values in human being are able to cure mental illness and guide mankind to a better life [15] .

Basically, positive psychology could be divided into three levels: individual level, group level and social level. At individual level, the research focuses on individual, it attempts to dig positive features in human nature, such as encourage, creativity, wisdom and tenacity [16] . Then, it refines power from positive features to cope with problems in daily life. At group level, the research focuses on organizations and institutions. It seeks to build a positive working environment through positive coach and positive therapy. At the social level, the research focuses on social system. The objective of it is to provide a positive social order for achievement of well-being, cultivation, justice, tolerance and sustainability [17] . Clearly, there is a progressive relation in three levels, and positivity is the main axle which links them. Figure 2 displays the specific contents at each level and the whole relation among them.

Subjective well-being (SWB) is one of most important notions in the theory of

positive psychology. It refers to a self-reported measure of life quality and life condition. In 1984, Diener developed a tripartite model of subjective well-being, the model pointed out three distinct but usually related components of wellbeing: frequent positive affect, infrequent negative affect and cognitive evaluation such as life satisfaction [19] .

This measure is highly dependent on individuals and it covers most of factors of well-being, including both body and mental condition. Due to the comprehension of subjective well-being, it becomes a significant standard of active aging. Thus, as an indispensable component of positive psychology, subjective well-being bridges positive psychology and active aging.

According to the World Health Organization, health means a well condition in both body and mentality [20] . Due to the deterioration of human physiology and the impact of negative pressures in daily life, many old people have physical and/or mental problems. With the increase of old population, the number of people who have physical problems and/or mental illness also rises. This causes a serious challenge to subjective well-being in the whole society.

Normally, the deterioration of human physiology and mentality has negative impact on daily life and social participation of old people. In some cases, the physical or mental problems not only cause high pressures on old people, but also on their families. Hence, with the deterioration of health, the olds usually bear huge psychological burden, and the burden further challenges their mental health. Through this process, the life satisfaction of old people is declined.

Thus, health could be considered as the basic to achieve subjective well-being. Unhealthy condition will cause burden and pain, and further decrease subjective well-being of old group. On the contrary, healthy condition will guarantee the body without pain and the ability to regular social activities, the subjective well-being could benefit from these factors.

Among all the factors which affect subjective well-being, social participation enjoys a significant status. It greatly devotes to increasing the healthy level and satisfaction of the elderly.

A survey in China proved the important role of social participation in subjective well-being. In 2012, Wang conducted a research to understand the impact of social participation on mental condition of the elderly, he investigated 100 old people in several communities and the pointed out 79.2% interviewees agreed the positive role of social participation in their daily life [21] .

Indeed, according to Social-role Theory, everyone acts a social role. The social relation from one to another is based on the interaction between different roles [22] . However, many acting work stop after retire. Thus, social participation could help old people continue their action in retired life. This offers the elderly a sense of belonging and a sense of being capable. Through this process, old people are able to sustain their social values and realize their existing meaning, the subjective well-being benefits from these positive feelings. Also, Active Theory supports the positive connection between participation and subjective well-being. According to Active Theory, old group still keep their psychological link with society after they retires. The disappearance of this link could cause many mental problems that decline the satisfaction of old people, such as autism, aphasia and amnesia. However, participation helps to sustain this psychological link. Thereby, participation plays a positive role in improving subjective well-being.

Welfare also has positive effect on subjective well being. To understand the link between the rank of subjective well-being and the level of welfare among different countries, this research employs World Database of Happiness (WDH) and Forbes Ranking of Happiness to conduct a data analysis, Table 2 displays the outcome of finding.

According to the World Database of Happiness in 2017, most of countries from rank 1 to 10 were Welfare State. The Forbes Ranking of Happiness in 2015 also provided the same evidence.

Clearly, the top 4 countries in Table 3 are Welfare State. Over 68% people enjoyed happiness and the average satisfaction level is between 7.8 - 7.9. According to these two tables, welfare positively affects subjective well-being. Thus, welfare could be considered as a significant step to achieve a high level of subjective well-being, an advanced welfare system is beneficial to subjective well-being.