My daughters were 17 and 14, flew with Singapore airlines and I signed a waiver that they did not have to be minors. This would have meant them sitting in a supervised room at Changi for the duration of the transit time between flights (6 hours). When they reached manchester airport however due to a duty of care the airport would not let them through until their picking up Uncle had made himself known.
I have travelled next to UMs internationally, they are well looked after and they never seat an adult male next to them, if they are on their own they will try to leave a seat between them and another passsenger. The UM is usually boarded first and stays on the plane until escorted off to either the immigration or who ever is supervising them between flights. They are escorted right through the process until delivery into the presence of the agreed person who has to show exact ID that you have stated in the forms ( beware a spelling mistake!). So until the UM reaches the relatives regardless of redirections or delays, they are looked after and fully supervised until that point.
Many children fly internationally as UMs these days. I would say the age is right if your child is happy to have an adventure without you. Some 12 yo are clingy some 8 year olds will be fine with it, particularly if they have flown before and are familiar with the process.