To get a HKSAR Passport, a person must be 18 years old. A minor can get HKSAR passport, if one of the parent is holding HKSAR Passport. ( I may be wrong)
I don't think it's that simple. Hannah Wilson was denied a HKSAR passport not because her parents were British but because British rules don't allow a minor to renounce their nationality (which is required to be come a Chinese national). Otherwise she would have been granted a HKSAR passport as a minor I believe (she subsequently did renounce her British citizenship and is now a HKSAR passport holder).