Firstly, he signed a new deal because he is easily the best paid keeper in the world, possibly top 20 footballer too. I think he is attached to the club but money talks.
Secondly, age in football is not always the best variable. He started young, 18, and has been playing first team since then. That is already the average playing time for a keeper. Most keepers cement there spot between 21 and 25. De Gea bloomed early. Only some keepers have played from a young age to a old age and at the top level. Buffon and Neur come to mind. Look at Joe Hart, was Englands number one, great keeper but career spiralled downwards quickly at 30.
Now I'm not saying that will happen to De Gea, and I hope this 2 year blip is just a blip, but he should never think of himself as safe. I would love Henderson to come back and challenge him for one season to see what happens. We could get a new De Gea out of it. Look how Shaw reacted to Williams emergence.