When flying alone what id does a person under 16 have to have?

Your solo qualification, which you obtain through training from a flight instructor at an aerodrome, and in most country's, the lowest age for powered solo is 16 but you can do it in ultra or microlights from something like 13 or 14 or unpowered aircraft (gliders) from 15 onwards, that's the bare minimum to fly solo though is to have gone solo once before and you have your A certificate or you fly under supervision (from the ground) of a fliying instructor till you have your A certificate