I felt my oldest at 17, 20, and 24. At 17, I sat on a swingset
  late at night in a public park when no one was around and cried.
  It was the end of my childhood I thought. Coming soon. Came
  home, wrote a letter to my future self dated to when I turned
  42. At 20, I walked around going, "I'm not an adult. I'm not a
  teenager. What AM I? At 24, I sat at the beach while some
  friends went late night swimming. I ran the grains of sand
  through my fingers. Had a big nihlist moment. I wrote about it
  later on when I got back home to my computer. It was 1996 and I
  said to my online group, "I can see myself in 20 years, lonely
  and suicidal. I need to find a purpose." So, now it's 2016. 20
  years later. I'm not lonely and suicidal. In 2014, I was 42, I
  opened up the letter from my 17 year old self on my birthday. I
  became the man I wished I'd become, just in different ways than
  my 17 year old self was assuming, but I matched all of my dreams
  and I'm continuing to. Best thing that helped me (and I can't
  say it'd help everybody, just me) was when I realized that I was
  comparing myself to how I was at younger ages. When I was 17, I
  was an "old 11 year old". That's what I cried over. When I was
  24 comparing myself to "me at 14/15". Too old to be a teenager
  anymore. Once I realized, "I am the age I am and that's good no
  matter what it is" and stopped comparing myself to my past me's
  and the long gone social situations that don't exist anymore
  (school, old friends, old habitss, etc), it was freedom. I'm 44
  years old now. Every day is the best of my life because I'm
  always at the peak of it. I've attained a height never possible
  in my entire personal history. I'm 100% complete at every stage
  and every age. There is no comparison between them. So, there
  you go. I can't say this applies to everybody because I've only
  ever been me and only ever experienced "life as me". But I share
  anyway.