Social Challenge: Meet the senior citizens of world.

The World Health Organization (WHO) classifies human ages into the following categories: 

Young: 25–44 years oldMiddle age: 44–60 years oldElderly age: 60–75 years old
Senile age: 75–90 years old
Long-livers: After 90 years old

Aging populations can cause a number of problems, including economic and healthcare costs, and social support needs. 

Solutions include improving healthcare, increasing social support, and encouraging older people to stay in the workforce.