Most states do. It's called the proper use clause. However, many manufacturers state that three year olds (or even one year olds) can use a booster. It is illegal, thanks to the proper use clause, for an 11 month old to be forward facing in nearly every seat.
However, I wouldn't expect a cop to know the differences in the seats (the one without a one year restriction for forward facing is the Comfortsport, the Cosco backless boosters state one year and 30 pounds). And it's hard to tell how old a child is, parents lie, and they don't have a scale to weigh them. So unless they're doing a carseat stop (sometimes with click it or ticket, sometimes on their own) they're not likely to pull anyone over.
I also don't know if carseats/seatbelts are primary or secondary offenses in most states. If it's a secondary offense they can't pull you over for it.
Wendy