The Jedi master has aged
Yoda is an important character of the Star Wars saga. He appears in the first as well as in the second trilogy taking place about twenty years before. He is one of the greatest Jedi masters who ever existed and he sat on the grand council for several centuries. When Obi-Wan Kenobi wants to make Anakin Skywalker a Jedi, Yoda is the first to see that there is something dark and dangerous in him. In the first trilogy (taking place twenty years later), Luke Skywalker begins his Jedi training with Obi-Wan Kenobi but he dies in battle against Darth Vader. But he returns in spirit form to advise Luke to go to Dagobah to finish his training with Yoda who took refuge there after the fall of the Jedi order. If he seems at the beginning to be a bit senile, he will eventually teach Luke everything he needs to know before he dies.