I surely hope not. I'm 29 and still the happy, hopeful person I was at age 6.
I mean, when life events happens like the death of a loved one, or suddenly "too old" to wear certain clothes - does this make people naturally bitter, angrier and jealous?
Again I hope this is not true. I don't want to leave my 20s when I turn 30 next year and spent the rest of my life in mental torture.