Well, unless medicines are going to be spooned into the child's mouth by a pharmacist, there is always an element of "but the parent could ignore it" in any purchase. The point of the age range is to give the adult buying it the necessary information. If it didn't say anything on the bottle, then the 99.9% of parents who are careful about what they give their child wouldn't have that information.
The 0.1% of parents who will ignore the age information shouldn't mean the rest of us don't get told what age it is suitable for!
Stronger medicines have to be bought from a pharmacist who will check that the parent undrestand what age it is for and why that matters. And stronger still has to be on a prescription so the doctor has a chance to make sure the parent understands.
Unless you want to make Anadin and cough medicine presription only, or served up by a phramacist like methadone, you have to trust people to pay some attention and care enough to not harm their child.