at do you think things might have been like for Levi and Dominic if they had met in their 20s, both individually and as a couple(assuming they get together)

Well, Dominic was traveling around the world with the Army Rangers for most of his twenties. That wouldn’t preclude starting a new relationship, of course, but it would make it a lot more difficult, especially since Dominic wouldn’t be able to be open about it (Don’t Ask, Don’t Tell was in effect then).

The complicating factor, though, would be that Levi and Dominic wouldn’t like each other if they had met earlier. I mean, they didn’t even like each other when they met in canon. Before Stanton mellowed Levi out a bit, Levi was even pricklier and less trusting than he is now; before Dominic left the Rangers and subsequently sowed his wild oats, he was a restless free spirit with no interest in commitment.

Without having been changed by those experiences, and without being forced to get to know each other through events like those in Kill Game - events which made them rethink their negative first impressions of each other, learn the important things they have in common, and admire each other’s strengths - Dominic and Levi wouldn’t have been friends, let alone a couple. So all told, it’s a good thing they met when they did!