When you're young, you can stay in relatively good shape in both form and function merely by getting out of bed each day. I was never "athletic" in the sense that I played a lot of sports but I did do other athletic activities, swimming mostly. I was in great shape and was the runner-up for 'best male physique' for our high school year book thingy (you know, 'best personality', 'most likely to succeed', that stuff).
I ate and ate like a typical 17 or 18 year-old male, nearly ate my parents out of house and home, and despite not being an avid weight-lifter or athlete, I was pretty muscular. Around 24, that all changes. They told me it would, but I didn't believe them.
My adolescent hormone charged metabolism gave-way to an adult metabolism, and since I was a lot less active than I was at 18, I started gaining 3 - 4 pounds per year. But I just accepted it because I was no longer in high school and I left all that big popularity contest BS behind when I graduated. The older you get, the less your peers look as though they just graduated from high school, too.
I say I'm going to lose some weight and hit the gym, but you know what? I won't because I don't really give a damn, who the hell do I have to impress? I'm comfortable with it.