My 10 yr old twin granddaughters are exactly 5 feet tall and they seem to fit the 3-point belts in my Cadillac and Blazer pretty well.
When I was in an accident a number of years ago the seat belt did more damage then anything else.
Cars built before 1965 that didn’t originally come with seat belts are exempt. EXCEPT, at least in NY and FL if children 12 or under are riding in them. My lovely daughter was 7 when I bought my 64 Chevy. It has the factory installed seat belts which were an option then, otherwise I would’ve had to install them.