So did Steve create another timeline in which he never became Captain America, never did anything heroic, never got frozen in ice and woke up in the present? A timeline in which his friend Bucky is still brainwashed and Sam "Falcon" Wilson is still some guy running in a park? And HYDRA was allowed to just keep doing its thing? He just blithely enjoyed his retirement, knowing that all of those horrors were some other universe's problem to solve? Or did he age as a civilian within the same timeline where he was Captain America, meaning he still stood by while a ton of other bad shit happened? There's no pretty answer here.
That's always the problem: These movies want to have it both ways. In The Butterfly Effect (which is most notable for losing the Best Thriller award in the 2004 Teen Choice Awards to the Texas Chainsaw Massacre remake), Ashton Kutcher's character has the power to travel between different stages in his life and make changes that affect the plot. The effects are only noticeable to him, except when the movie suddenly needs him to prove his abilities to someone to get out of danger. He travels back to his childhood and creates Jesus scars on both his hands. He then goes back to the present, where one person somehow has the memory from when the scars weren't present, at which point the hamfisted religious imagery takes over.
Something similar happens in Looper, where it's stated that any physical damage to a person's younger self is always present in their older bodies ... until it's time for a dramatic scene that shows someone's younger self being mutilated and their older self losing limbs in real time, the person screaming as their fingers vanish in front of them (rather than their memory calmly adjusting to having lost them years ago). Time travel offers almost limitless possibilities, but somehow writers almost always wind up painting themselves into a corner.