I use clippers on my DS' hair and have since he was about a year (when he quit holding still so I could cut by hand). His hair grows FAST and I end up cutting every month and a half at most. Do you have the extension pieces with the #s that make it longer than a "true" buzz? I usually do a 7 on top and a 4-5 on the sides for my little guy but he has very blonde hair would look bald if I cut it any shorter plus I like having some hair to play with still.
Anyway....no, like everyone else has said, I don't think 18 months is too young. It's hair, it grows back.
Well, I do have the "extension" kit, and I should have used it!! LOL!! I just give DH a straight buzz, and I did that to Aidan. Oh, he looks funny now!!!!!!!!!!!!! Oh, well, in about 3 - 4 weeks he will look really cute. In the mean time, I have a skin head!