i base these on the fact that Thedas seems to have modern ideas when it comes to ages rather than medieval where a 14 yr old would be an adult, that many are basing things on.
Cullen - Based on alistar being born in 9:10 and being a templar in training in 9:30 we can assume one cannot be a full fledged templar before the age of twenty leaving cullen at the very least 32 (i'd say more 36-38)
Leliana - she's 26 in origins according to the toolset so 37-38
Josephine - don't know enough about her hazarding a guess that even given her knowledge shes still youngish 24-26
Cassandra - late 30's early 40's (based on the Idea that she was 18-20 as a "young seeker")
Varric - born in 9:02 so he would be at least 40
Solas - he's bold and an elf who knows, his views seem to convey wisdom and weariness i'd say mid 40's or older.
Blackwall - again complete guess early 40's to mid 40's
Vivienne - having not read masked empire i can only go on her looks and position, enchanter of the imperial court doesn't sound like a job of the young. I'd hazard late 30's to early 40's
Dorian - early 30's i'm thinking the traditional dynamic of the student surpassing the teacher in skill and wisdom is going on with him an alexius (with alexius at least 55) rather than the upstart who's just more powerful.
Iron Bull - getting a background on iron bull in his ben hassrath role being "burned out" in 9:40 his scar's the size of his horns and that Qunari live longer than most, combined with his wavering of Qunari rules i'd say he's the oldest of all 50+
Sera - all the hallmarks of youth arrogant, stubborn, selfish and all about the here and now early 20's