SPOILERS for Avengers: Endgame ahead
Captain America could always lift Mjolnir in Avengers: Age of Ultron according to Avengers: Endgame directors Anthony and Joe Russo. Up until its destruction in Thor: Ragnarok, Thor's magical hammer Mjolnir was one of the most powerful weapons in all of the Marvel Cinematic Universe. However, thanks to an enchantment by Odin, only someone worthy would be able to lift it. Thor was originally deemed unworthy during his first solo movie before regaining his worthiness.
The debate over who was and was not worthy to lift it was brought to the forefront in Avengers: Age of Ultron, as an entire scene is dedicated to the Avengers each trying to prove their worthiness. Iron Man and War Machine can't lift it with their combined strength; Hawkeye tries and fails; and Black Widow refuses. But, Captain America's attempt does see him slightly move the hammer, and Thor noticed. This was taken as a tease that he is the most worthy of the group to hold it - up until Vision picks it up later on in the film - and has left fans wondering if he was actually worthy then or if there was something holding him back.
This conversation sparked back up after Avengers: Endgame hit theaters and showed Captain America picking up Mjolnir and showing complete control over its powers during his fight with Thanos. During an interview with the Happy Sad Confused Podcast, the Russo brothers were asked why Cap is worthy to lift the hammer now but not during Avengers: Age of Ultron. Anthony explained that in their minds he always has been able to do this, but he elected not to before.
In our heads, he was able to wield it. He didn’t know that until that moment in Ultron when he tried to pick it up. But Cap’s sense of character and humility and, out of deference to Thor’s ego, Cap in that moment realizing he can move the hammer, decides not to.
Prior to this explanation by the Russo brothers, many had theorized that Cap knowing that Bucky was the one who killed Iron Man's parents was the reason he was deemed not worthy. Covering up such a major piece of information and hoping that Iron Man would never find out is a very good reason for why Mjolnir budged but wouldn't let Cap completely lift it in that moment. But, since he was forced to admit this reality in Captain America: Civil War, he would then be worthy in Avengers: Endgame.
This is not the case, at least as far as the Russos are concerned, and they use Thor's followup line of "I knew it" to the moment in Avengers: Endgame as further proof that Thor knew Cap was worthy all along too. Since this is their intended explanation for the scene, then this is probably where Marvel Studios as a whole stands. It makes for a fine explanation, but does eliminate any ties of Cap lifting Mjolnir to his larger arc, which may be somewhat disappointing for those who liked the Bucky/Iron Man rationalization instead. That said, Cap picking up Mjolnir is a great moment of Avengers: Endgame no matter what the explanation is for his worthiness now or in the past.
Source: Happy Sad Confused Podcast
