Some of the 50's are really small (Suzuki for example). My 7yr old had a DS50, which uses the same chassis as the 90's, and honestly that 50 is pretty peppy!. My son is a short guy so he has been able to grow into it, and should be able to get atleast another year out of it with some mods. We have beat on that quad for 2 yrs and its held up great! Our next step will be a 90 though, as we pass the 50 down to our youngest. Our daughter ran a Hetrick build Kaesa 90, man that thing was wicked!!. As Rupp said, you can never go wrong with an Lt80, pure classic. Whatever you do, dont buy Chinese, they will only waste your time and money. Take the little guy around to some dealerships, let him and you get a feel for the sizes of different quads, as theres alot of variences.