We switched DS to a booster and seatbelt when he turned 4yo - its not only based on height but maturity level. What carseat do you have? Sometimes the five points turn into boosters.
Plus, if you know he won't unbuckle himself mid-drive, you should be fine.
Agree. Its based on weight/height, not age. There is a website that tells recommendations, but sorry, it escapes me at the moment. You can also google it
My DD is six and we are about to switch her. I wanted to make sure the seat belt would fit correctly. She has been doing the harness herself for as while.
DS is tall for his age (always the tallest in his class so far) and he outgrew the height and weight limits for the Graco Nautilus somewhere around 5.5 years old so we switched him to using it as the high-back booster.
Another thing to consider is to make sure you aren't going over the weight limit for the latch on your car. I learned about it on once of the message boards, otherwise I wouldn't have known. I think sometimes the weight limit for the car can be less than the seat.
It's also bone maturity. The belt becomes okay around age 5. My two oldest were okay around 5.5/6 ish years old, they weren't squirmy. My 8 year old (about to be nine) transitioned at 7 - she was in a booster at 5, and no where close to ready. Caught her standing in her booster with the belt wrapped around her neck twice, pulled off the road and bought her a new seat. My sanity needed it - and wasn't the first time she'd done something like that. And my six year old is still harnessed because the 8 year old gets her riled up.
Re: Kids that are 6+ years old
Married DH 7/30/11
CSC arrived 5/7/12
CHC arrived 6/2/14