“Whosoever holds this hammer, if he be worthy, shall possess the power of Thor.”
Nobody other than Thor had been able to lift Mjolnir in the Marvel Cinematic Universe, despite many trying. The closest anybody came was Captain America ever-so-slightly budging the hammer in Avengers: Age of Ultron. That was until Avengers: Endgame, when Cap wielded Mjolnir, pummeled Thanos and provided us with one of the best moments of any Marvel film. So what finally made Cap worthy? Whilst speaking with the Business Insider, Endgame directors Anthony & Joe Russo revealed that he had always been worthy.
I think in Age of Ultron he may have not picked up the hammer in order to not embarrass Thor.
That’s the way we think of it.
In a separate podcast interview, whilst speaking about the scene in Age of Ultron the directors added “Cap’s sense of character and the humility and the sort of deference to Thor’s ego, Cap in that moment realising he can move the hammer decides not to.”