Yeah, you want to have it capped (99) before hitting the level 20+ zone. You gotta go to the city to buy the book to be able to skill beyond that, though. I think it's like 18 or 21k? I duno, I haven't bought it yet.
Mine's only like 60 something but you need to extract as much as possible. Each new zone just gets higher and higher so you need to keep up on extraction otherwise you'll be wasting time going back to leveling it up when you actually need **** for crafting.
But if you want to come back at a higher level that is fine because mobs won't aggro you if you're 5 (I think) levels higher than the mob.