I’ll answer in reverse order bro
1) why not under 21 (personally I’d say 25 - unless one is aiming to compete)
Basically because at that age most of the gains you can make on aas can also be made naturally with the right diet and training. Doesn’t matter what you take the body can only create new lean mass so quickly and at that age (for 99% anyway) the natural hormone balance you have would allow for most of that muscle building to occur. The other reason is that very few people under 21 (note - not all and I know there will be exceptions) have the right displine or training experience to get the most out of aas.
If your going to mess with you natural hormones you want to maximise the benefits and minimise the negatives, the younger you are the more chance that won’t happen and you get kids creating long term health issues (needing trt in their 20’s for example) for less than optimal gains.
2) what difference does it make
Depends on genetics and the stuff above (diet/training etc). But a big difference if those things are on point imo. I’ve only run orals to date but I’m 36 and had basically plateau’d (well progress was very slow anyway) for a year. I’m a qualified PT and lost 140lbs 5 years ago so whilst I’m always learning I’m also pretty confident I had a pretty decent handle on training and diet.
Basically over 2 cycles (I.e around 9 months) I added over 24lbs lean mass (bf is slightly lower), back squat is up 60lbs, bench up 55lbs and deadlift up 90lbs
I have no idea where the ceiling is, I’m going to pin test for my next cycle but I’ll eat right, train hard and based on previous experience expect to make more lean gains.
I’m glad I waited till I had the experience to do it right but I don’t regret running anabolics at all - allowing me to do things I simply couldn’t do natty