Ds is 6 1/2 yrs old and about 49" tall and 52 lbs. he is in a fn 85 but doesn't have that much space left, he slouches a lot so it's hard to tell. If he sits up as straight as he can he is about even with the top slots but he doesn't maintain that posture. Unfortunately he is no where near ready to ride in a booster. I have one for emergencies but unless an adult is back there with him, it wouldn't work. I have a hard enough time with him not messing with the harness staps/buckle. I'm really starting to stress about what to do when he outgrows his seat!