Just watched age of ultron in preperation and my God it does not hold up too well, probably the worst one to watch beforehand. But I'm in the end game now, nap time then film!
On the contrary i think it holds up better than most of the early films including avengers 1. Knowing the payoffs of endgame due to the A LOT of setups in AoU make it that much more satisfying. The dead avengers scene cracked shield and all under invading chitarui, Clints family, Starks vision and journey to stop it. His mind set on preparing for a moment he knows is destined to come. "That up there, thats the endgame. How were you guys planning on beating that?". "Together". Its so so good for those seeds to grow. Even Banner's struggle with the hulk in the film. The mindstone with the vision.. So much that later paysoff in IW and Endgame that make the AoU so good on rewatch as you know where it leads. Nah son, age of Ultron is a fine wine that gets better as the MCU progresses