Indeed, although he is often cited as resembling a young Luke Skywalker, he really resembles a younger Mark Hamill, as young Luke Skywalker was already featured in the original Star Wars trilogy.
Middle-aged Luke Skywalker is indeed the chapter in the Jedi Master’s life that we know the least about.
As Sebastian Stan told Esquire, the time between Return of the Jedi and The Force Awakens is the period he could explore if he were to become Luke.
The events following Return of the Jedi were once chronicled in books that are no longer considered canon.