You can work on speed, explosion and balance. It’s not unusual. And at 18 most kids who work in these things do get more athletic. Look at how hard NCAA football players train. Now I believe each individual has a different ceiling in the level of athleticism each can attain, but I’d venture to guess that few 18 year olds have maximized to their ceilings.
I guess that depends on what you mean by athletic. There are kids that are physically gifted and who get stronger each year. Some of those same kids are just not as gifted around the rim or as light on their feet on the football field as others no matter how strong they get.
Getting stronger does not always mean you are as athletic or agile around the rim no matter how much you trained. That’s why many players no matter how strong play below the rim.
That’s what I meant. You can’t “teach” that.