Given the fact that DA is actually sort of medieval in its setting and take f.e. the city elf origin where your character is about to get married after putting it of for quite some time 18ish seems to be about right. I also think all Wardens have a sort of older, more serious air about them, even Carver and Bethany, or especially them and this may even count for Anders. I mean they actually don't have the time of life, they know what their future holds, and how dread their path is.
yeah 18-ish seems around right for the origins, to me. it’s a hard knock life. they all seem older, but yeah, they go through some crap to make them age beyond their years.
and it’s the norm, really. i’m sure most people get married and have 5 babies before they’re 23. never know when you’re gonna get eaten after all by darkspawn or whatever monster is out there.