I'd say mid to late 20's your natty test starts to decline in your late twenties... that would be a good way to go... but as long as you've been training for a good while and your body is hitting it's nat maxes I'd say 22-24 would be good if you're training and discipline is ready!! and some say younger is ok ... but we're not talking about when is the earliest acceptable age, we're talking about abou when is it "best" to begin