About 2 years old.
i saw a 4 year old jumping up and down for mommys boob a few years back . it just looks real bad .
ewww sounds really bad!
I think if they are 18 and still breastfeeding, this is a sign of extreme attachment. I would try to break it off well before they are ready to leave the nest for college. Otherwise they will go into withdrawal and desperately seek out another source in college.
If someone is still breast feeding their kid at 9, there's something wrong with the Mom. It's not about breast feeding at that point.