An amazing topic idea. I've written about this a bit in my blog. I've got some things I wish I did differently, and some things that I did RIGHT back then that I learned was "wrong", then realized much later in life that I had it figured out.
THINGS I WOULD HAVE DONE DIFFERENTLY
-I would've started lifting earlier, instead of listening to the nonsense about it stunting your growth until you are 13 (which was the hardfast number going around at the time, and nowadays I hear anything form 14-18).
-I would've started with 5/3/1 right away and quit worrying about "beginner routines". I already had a solid foundation of athleticism, even as a fat kid, and the heavy work paired with the assistance would've really given me a sustainable plan to follow for years.
-I would've learned what actually was and was not good food at ate accordingly. I had it pretty sweet, and my dad made breakfast and lunch to order for me everyday, whatever I wanted. I ate a lot of good things, but I also ate a lot of junk that was disguised as good things. Bagels, juices, lots of carbs I didn't need. And then my mom did a great job of keeping the house stocked with amazing junk food (again, out of love rather than neglect). I ate a lot of stuff because I could get away with it, but I'd be so much further ahead if I figured it out then.
-Spend more time learning about the mind/muscle connection. Getting that figured out early would go a long way.
THINGS I WAS DOING RIGHT AND QUIT DOING BECAUSE I WAS STUPID
-I didn't worry about overtraining at all back then, and amazingly I never got overtrained. I'd spend hours a day training in some capacity, between lifting, cardio and martial arts. Then, a while later, I found out about the overtraining boogeyman, trained significantly less, and got less results. Huh, how funny.
-I thought getting bigger was just as important as getting stronger back then. It seemed like you should do both when you lifted weights. I then learned about the hypertrophy rep range vs the strength rep range, and how sarcoplasmic hypertrophy was just worthless show muscle and that I wanted myofibillar hypertrophy and should only train in the 1-5 rep range to get stronger (because wanted to look better was for wimps). Holy shit did I get stupid when I got older.
-I thought someone should be able to be strong AND fit (and god forbid, leanish). I watched a lot of action movies, read comic books, and idolized these characters that were strong and fit and looked good. Then, later, I learned that you had to EAT your way to strength gains, and put on 100lbs and not care what anyone thinks of you because you're getting jacked, and not being able to walk up a flight of stairs without getting winded was something to take pride in. Again, how was I so much smarter when I was younger?