A backless booster seat is not very different from a high back booster. It would be incorrect to state that age does not matter, however, it is a matter of weight and height rather than age as such.
The key difference between a backless booster and a high back one is that the former only positions the lap belt and elevates the child. The shoulder belt is left unguided. Additionally, high back boosters have side impact protection backless boosters lack.
You can find out more about the weight and height requirements if you follow this link.