Seems to be 18 with Ryanair.
However for the last row of seats (exit row) they seem to have a different policy. I was traveling back from Alicante when I went to sit in the last row of seats, I was asked for my passport and ticket as soon as I sat down and was told to move because I'm only 16. Soon after, boarding ran late and they stuck 3 young children in the row I went to sit in, and their parents in the row across from them.
The way I looked at it was... In an emergency is a 7 year old better than a 6ft 4" 16 year old who's currently building hours for his PPL?