DS was 13 or 14 the first time he flew solo. It was fine, he was a seasoned traveler but as long as she has flown before she should be fine, or just get a gate pass.

Both my teens really enjoyed flying solo. At 16 dd had a trip with plane change and terminal change which made me nervous but we looked at map of airport ahead of time so could talk through what she had to do and airports are pretty well marked. I’d def let a 15 year old do this trip, especially if she can fly direct.