Pretty much what the title says. I have 2 characters who are technically the same person, just from different universes, but for multiple reasons they have to be different ages. Can this make sense? like, could one of them just have been born earlier? The science can be extremely soft and not serious at all. Time shenanigans could also work but I'm not a big fan of time travel and would prefer to keep it out of the story
Alternate universes is a fine way to deal with time travel.
The old version of me is from my future in an alternate universe. In that future he has been trying to open an anchovy restaurant. He shares many of my experiences, our different universes being similar. But he has had more life experiences than me. He has to have - he is older! At least 1 year and a half years older than me, the freaking ear hair monster that he is.
He recounts the hard life of the anchovy restauranteur. He suggests that if he could have done things differently, he would have opened a smelt restaurant. In my own timeline I do exactly that, thanks to old me who showed up to coach me. When my old self goes home to his universe in the future, it is back to his cold dark anchovy restaurant. I did not have much good coaching to give because he already knows all my tricks. But at least I listened, sort of.
A fine way to have time travel is to have it actually be hopping between universes, with the arrived at universe the one where you showed up in the past to meddle with things. If you sex-change your grandpa in that other universe you will never be born in that universe. Then when you go back to your own timeline you pickup where you left off there in your own timeline. Or maybe you are older by the time you spent in the other universe. In any case your Grandpa there is still male and your time meddling took place elsewhen.
You could make time pass slightly differently in each universe. If the difference was small enough, there wouldn’t be any noticeable effect. By the time your story takes place, time in the alternate universe has diverged by the number of years you need for your narrative. From the perspective of each universe, time would proceed at an identical pace. Even crossing into a different universe wouldn’t matter as the speed difference would only be measurable over billions of years. A negative aspect of such a system would be that it would technically be a form of time travel between two fixed points. There wouldn’t be any time travel within the same universe, just between each universe. It avoids the messy implications of stereotypical time travel technology, but fails to prevent travelers from the slower universe from taking technology from the faster universe and leapfrogging their technology forward by x-years. It wouldn’t impact the faster universe, but both universes would diverge more drastically from that point onward. Not really much of an issue if you only want an age difference of a few years, but more of an issue when talking about decades. As long as the person from the slower universe makes the choice to avoid such contamination, this is a non-issue.
There might be some other issues such as gaining foreknowledge of upcoming events. If the two universes have diverged by five years, then the person from the slower universe might see the emergence of a disease like Covid, and be able to gain access to the vaccine. Returning with that knowledge, he/she could sell the vaccine to a pharmaceutical company and prevent millions of deaths. Something to think about.