[AstralStorm]

It's mostly that very few things change quickly enough anymore.

When you're a kid, you change schools, pass exams which serve as time markers, have various important lifetime events. As a young adult, there are a bunch of those too.

You know you're getting old when you start measuring your life by events related to your children or grandchildren.

Older people have fewer of those, especially ones in a stable situation.

[ejolto]

I think so too. I'm in my thirties and have lived in three different countries in the last four years. Time feels really linear and it doesn't feel like n months/years have flown by.

I try to always do new things so I don't wake up one day with ten years flown past me.

[sibeliuss]

This is it. It's the same as when one goes on vacation to somewhere new and time stretches way out. As one gets older and time awareness becomes a thing a choice presents itself: how do you want to work with your days? Novelty stretches time, routine contracts it. Time is very possible to "work with", but how well that relates to responsibility is another matter.