Air Berlin
The kind hearted soles at Air Berlin accept UM's.
I think the minimum age is 6 if I remember correctly, although I've seen all ages. I dispatched one last night with a 16 year old "UM" on board which made myself and the crew laugh!
Have seen some very young UMs too. At Air Berlin they will have a Servisair employee escort them and stay with them all the way from check-in to the aircraft door, where they'll be introduced to the number 1. On the return journey the same procedure happens in reverse, and the parent or guardian will need to show ID before the child is handed over.
Seems to work for Air Berlin as we have many UM's through STN daily, I once saw 10 on one flight!