I am 30 and still can look 18 when I bother to trim up a bit.
Naturally, I lagged behind most of my male peers in physical development. I am still building up muscle and tone now to the point of what most other folks with athletic builds looked like in early high school.
Slam me if you will, but one thing that really helped was those magnetic balance bracelets. 5 bucks on eBay... They train you to somehow coordinate your whole body while performing work. When I first put mine on I went to my back hillside and started picking up entire 30ft tall trees and throwing them down into a pile, and I did not slip or strain myself once! You need to wear them for at least a year for them to have semi-permanent effects, which mainly consist of your brain learning to fire sets of muscles in the proper sequences to stabilize you and help you lift and move things better.
Protein shakes also help, especially immediately after you've done heavy work. This will at least help your body recover to where you a) won't be in pain for days and b) won't be as likely to burn off what little precious muscle mass you have.
After you've built yourself up somewhat, you can actually rest easy, just walk around a lot and eat at least 2000 healthy calories a day if you are 6ft or taller. And, sleep, sleep, sleep!
_________________
AQ: 42
aspie-quiz: 151 / 47