My philosophy
[*]You are middle aged the instant you say "I won't" for simple tasks.
[*]You are old the instant you say "I can't" for simple tasks.
[*]You are young until either of those occur.
For example: I know several people who are in their 80s, who still do martial arts, who still take dance lessons, who still go out on the town, etc. These people are young to me.
For example: I know several people in their mid 20s who won't go downtown because bars and entertainment are for the college kids. These people sit at home day after day, night after night. Their life is basically over. They are middle aged, since they won't do things.
For example: I know some people in their 40s who say they can't watch a movie since their back hurts, they can't go out to eat since it is bad for their health, they can't see their kids at their sports/performances since it would require them to stay up past 9 pm, etc. These people are old.