I mean it's all in the past. You can't travel to 'present 2014' because all of 2014 is in the past. The present is 2023.
Here's what I'm saying: just time traveling to a previous year does not by itself create a new timeline. If it did, then this movie is just constantly creating new timelines out of every little thing. It would defeat the whole purpose of Steve's mission at the end, because a new trip through time would just mean new timelines, and obviously the movie is not intending for us to believe that he created more branches rather than less.
So as long as that's the case, then traveling to 1948 to marry Peggy wouldn't automatically create a new timeline because it's not actually changing any established history and it's certainly not changing Steve's past (past Steve is still in the ice). When the Avengers go to 2012 to take the mind and time stones and accidentally spring Loki free, and reveal the HYDRA and Bucky conspiracies a couple years early, that would create a new timeline because that's dramatically changing the past. But in 1948, as long as that year's Steve Rogers is still in the ice, and as long as a Captain America isn't running around fighting bad guys, and as long as Peggy's original husband hasn't been wiped out of existence because 2023 Steve Rogers married her first (and as the writers make clear, Peggy's husband was always kept unidentified because he was meant to be Steve from 2023), then the past hasn't actually changed. And if the past hasn't changed, then there's no alternate future to stem from that altered past.
He's old as hell at the end because 1948 -> 2023 is 75 years and that's what a 100-year-old guy looks like, not because he's traveling back from a year beyond 2023. There's absolutely no way that was the implication of that ending. Did anyone in the theater, watching that scene unfold, think to themselves "Oh I get it, he's so old because he traveled to here from the future"? I really doubt it.