I've got one where the woman is about 5 years older than the man. As mentioned, it depends a lot on the age of your adoptee, and which proposal they accept.

I don't see a trophy for divorce XD so maybe they don't get a divorce. I can however see one getting married a second time if the first spouse dies when the other is a relatively young age. The game guide mentioned that the next generation can take over when they are old enough and one of the parents dies. So I think if the children are over 20 or so, and the parents are not of child bearing years (I think sometime around 40 the woman can't have any more kids even under normal circumstances), there probably won't be any more proposals coming in for the Spouse who is single. Younger than 40, or no children old enough to take over the house, might make for a new round of proposals.