I've brought coconut milk through for my son who is 6. He has dairy allergies and some food issues that mean his only other option would be water for 20 hours. When I mentioned his issues they have allowed (from Bos and in Europe) is to continue with the milk boxes after (in Bos only) undergoing some additional screening. One time they rifled through my carryons thoroughly, and another time they had me open a box so they could wave their magic wand over it and scan the wand. (Was fine because my son was thirsty, anyway, so I just let him drink it after they checked it.)
It's not really about the age but the issue at hand in my experience. I don't mind the extra screening, so it's fine by me.