The gap between DAO and the end of DA2 is about roughly 10 years.
Which means if Bioware decides to stick with that kind of time difference, Cassandra will be 50 by the end of next game, practically a granny at that point and that is exacerbated if your Inquisitor is in his mid 20s because he will only be in his mid 30s.
Well 10 years is a very big deal for someone in their 20s, you are right. You are going through so many life changes at 20, and still figuring out who you are. You are in a different place mentally so there's a higher chance you would have little in common with someone in their 40s anyway, even if you found them attractive.
When you hit 30+ though, meh. Your priorities and perspective changes. A lot. You actually do think about the present more often. You appreciate things that are fleeting more, because you become more aware of your own mortality. You have lived long enough to see more people you know and care about start to get old, sick, or die. This is doubly true if have any children of your own, as you watch them grow fast and years seem to pass more quickly. Not to be depressing or anything.

But doesn't Cassandra's romance touch on this theme a bit? About life and romance being fleeting? I seem to recall a line where she talks about how she wants to pretend a bit longer. She doesn't seem to expect anything permanent at that point. Actually none of them seem to, except maybe Cullen and Sera, who you can marry. Maybe Josie.
When you are 20 and younger, everything is about the future.
I suppose a much younger Inquisitor could come to understand that and have a relationship with Cass...Given what they have been through and the whole danger and uncertainty of their lives. Settling down to have children could be the last thing on their mind.
As for Solas, arguably the oldest in the group, his romance does have that sort of fleeting sense to it. The age difference really doesn't matter because it's not about that.