[Pepsi]

This article has some mention of Cap coming back as an old dude

"If Cap were to go back into the past and live there, he would create a branched reality," Joe Russo told EW. "The question then becomes, how is he back in this reality to give the shield away."

Even for fans who had worked out the time travelling mechanics, Cap's presence at the end as an old man was a major plot hole. If he's an old man in the same space as Sam and Bucky, then how could he be in a different reality?

The implication from the Russos is he used his remaining Pym Particle to jump through not just time but also across dimensions to be in that spot at that moment, rather than having lived his life until then in the same reality.

Which answers all those questions about Cap going back in time and somehow not affecting everything that's already happened, such as Hydra filtrating SHIELD or Bucky being under the control of Hydra for decades, or, say, an alien invasion of New York.

In his new reality, maybe he stopped all those things.