As for kids, well, kids simply don't have a developed enough brain to make proper decisions about a substance as addictive as meth. The argument here is purely biological and doesn't really need a defense. I'll preempt the usual counter argument by saying that if we could quantify decision making ability, I'd be all for using that instead of age. But age is the best we've got right now.
As I said before, if it doesn't affect anyone else I don't really care what you do. Parents can do whatever they want (drugs or otherwise) if it doesn't affect their kids or other dependents. In addition, anyone with a job needs to consider their responsibility to their company. But hey, if there's someone where none of these connections apply, s/he can do whatever they want. I certainly wont judge.