DS2 is 9 years old. He's 53" and 70 lbs. He still prefers to be in a HBB, so in my vehicle, the vehicle he's in most of the time, he is. He recently outgrew his HBB that was in DH's truck and rather than buy another HBB (he's outgrown all but two), I replaced it with a backless.
It varies per kid and vehicle. All issues favorable (seatbelt fit, head support, child's maturity), I wouldn't really have an issue with a "responsible" 7/8 year old in a backless, if they had been riding (properly) in a HBB for awhile.
In order to be "ready" for an adult seatbelt, your child should (in addition to the legal bare minimum age/height):
Have adequate vehicle head support - either the vehicle seat or headrest/restraint to, minimally, the tops of the child's ears.
Be able to sit all the way back, and upright, without slouching, on the vehicle seat, and have their knees bend comfortably over the seat edge. And remain that way the entire ride.
Lap belt should rest across the bony hip/upper lap/thigh and not make contact with soft abdomen.
Shoulder belt should rest centered across the shoulder and chest.