My father decided at 62 that he was gay...he and my mother had been married 40 years at that point. I don't know the why of people shifting their sexuality...he had been a serial cheater with women for years prior...I just know that it does happen. They separated for a number of years but found it was too expensive to divorce. now in their elder years they live in the same house again like brother and sister.
I think maybe if it's going to happen, it's way better to happen when you are a young person and both partners can move forward (albeit not without pain) in their lives.

I wish you the best.