If you can get a fast-training 18-year-old, you can train a special ability and have him at 9* by the time he's 22, if you're prepared to watch a lot of videos, save all your green packs and invest them in him. You also end up training him a lot.
Once he's 8* or 9*, you can imagine that he gets MoM a lot, so that's quite a few skill points. Also, once they're 9*, the rate of progress for training is so poor that it's not worth doing, really. Probably true of 8*, too. Better to get them to 8* and then save the packs for the next season.
Anyway, suffice it to say it takes a lot of time and effort!