It happened to me last year, A few weeks to a month before I turned 20.
1: Most people live to be around 70 or 80. So if I was to live to 80, then 1/4 of my life was already gone.
2: Being 20 means you're totally out of your teens, no more "I'm a teenager".
3: All my friends around my age also grew up. We all got done with High school, some headed off to College right away. Most of us got jobs, and we just grew up. People I've known since like... 3 has gotten old.
4: It also hit me again when I found out one of my younger cousins is engaged. She's 17 now and getting married next year. Someone I've known all my life, grew up together is now getting married.
Lately stuff has got me to thinking. But I try not to think too much, if I die of old age then I still got a long life. No need to waste it thinking about how old I am.