Well, Cassandra is anywhere from 35-40 in age, so I don't see Vivienne being much older than that.
Yeah, actually most of the companions seem like they are going to be older.
Cassandra has to be in her mid-30's.
Varric is around the same (Wait, do dwarves and elves have extended lifespans in DA? How do I not know this?).
Cullen (if he's a romance and/or companion) is about the same. \
The Grey Warden looks more like late 30's through 40's.
Vivienne looks like she's anywhere from late 20's through late 30's.
Can't really tell with Solas.
DHMG looks like he's in his 30's to 40's.
In fact, the only ones who look like they might be younger are Sera and Scribe Girl (but even she could potentially be a bit older).
I'm liking this trend, frankly. It makes so much more sense for the characters to be older and more established given the international influence that the party will likely have.