Answer:
Maybe that was the peak of their existence. The time at which they'd accomplished most things, when they'd met the greatest people, when they'd fallen in love and fallen out.
It is harmless as long as they do not live in the past. Reminiscence is not a bad thing. But they have to remember to live in the present.
However, it's not the same for all adults.
Not everyone has had an eventful youth. For some, 'the time of their life' arrive at a later point. For others, it doesn't happen at all.
