How old do you have to be to buy laxatives?
I don't believe you have to be any certain age to buy them. I think anyone can do it. Just don't use them for something other than their intended use (short-term relief of constipation). Also, Milk of Magnesia works like a gentle laxative, with no stimulant effects.
The fact that you are asking that makes me think you are going to abuse them. They will seriously mess your body up. Don't do it.
You don't have to be a certain age, but just remember not to use them to lose weight. it can be very harmful.