Your experience is quite similar to mine. University was great, but apart from that my 20s were miserable. Although in my case not because I was single, but because I was mostly with my ex-wife.
Teenage years were very mediocre due to not enjoying school and the stress of exams, and also the...
If I could relive any decade again it'd be my 30s, definitely. Old enough to have established myself career-wise so having a decent amount of expedable income, but not so old that I couldn't still play sport at a decent amateur level.