Does age matter as much as size to you when it comes to transitioning your child from a car seat to booster?

My son is 2 1/2. He is very big for his age. He meets the weight requirement for a booster seat. (40 pounds) He is also almost 40" tall. His car seat is wearing badly. I need to get him something new. I am very tempted to get him a nice high back booster, but am finding it difficult because he is still so young. I don't know why his age is deterring me, though...I know for a fact he is strong as an avg. 4 year old. In fact the pediatrician told me he's the strongest 2 year old he's ever seen. What would you do?