If he was the 2nd most powerful Warden in all of Orlais 10 years ago, then he definitely should be 40+. I can't really see someone under 30 becoming Warden-Constable of Orlais. In Ferelden maybe, cause there were like 10 Wardens in Ferelden total. But Orlais has to have 100+ Wardens since they were never kicked out and should have been maintaining their numbers. He is probably the Warden-Commander of Orlais by now (In Calling it is hinted that W-C's have short tenures as they are typically fairly close to their calling by the time they reach the top), so that would add to the 40+ theory.
The Calling part will be interesting, but I'm more intrigued about the whole 'Warden-Constable' part.
We're not talking about a common Warden grunt joining the Inquisition, we're talking about the second in command of the Orlesian Wardens. Now it's even more interesting to know what made him to defect, since the letter in WoT and the picture that revealed his name (couldn't someone put it in the first post? or did I miss it?) make him look like a man who takes his responsibilities as a Warden very seriously.
In fact, so seriously (that grim look of him sells the deal, really) that his famous letter is an unintended source of hindsight humour.
Blackwall: "We must be ready to stare squarely into the eyes of oblivion!"
Warden: "Relax, pal, don't worry."
Blackwall: "What?! We can't relax! We're in the middle of a Blight!"
Warden: "Not anymore".
Blackwall: "I beg your pardon?"
Warden: "Killed the Archdemon already. Ferelden is saved".
Blackwall: "What? You stopped a Blight in less than a year?"
Warden: "I'm just that good".
I wonder if he met our Warden. He at least must have heard of him/her.