I started experimenting with pumping when I was about 16 (and before a mod goes and bans me I'll point out that I'm now a paid up adult, not an under-age poster!)
At that time I didn't know what I was doing and it'd have been fantastic if I'd had a resource like LPSG but unfortunately I didn't. It ended up more of a pleasure thing than a properly thought out development program.
My own opinion is that to get the maximum benefit from enlargement you need to do it during puberty, whilst your cock is growing naturally anyway. So in that respect I should have started when I was about 10 or 11 and carried on until 18. I know that in Arab cultures this is exactly the time manual techniques are used, but I think here in western society it would be a bit frowned on. Unfortunately anyone giving such advice to a teen would probably be seen as some kind of kiddy fiddler. I think a lot of teen boys are very pressured by penis size issues these days though, and will experiment with penis enlargement wether or not they know the facts. So through a lack of guidance many end up doing potentially dangerous or harmful things like taking dodgy pills. If I had kids I'd give them the info along with all the other sex ed stuff they need.