That's true but personally I don't pick an age for my protagonist I kind of think of them as an age that seems appropriate to the story. The Warden seemed to me to start quite young, about 18 or so. And I would say Hawk and Sheppard feel about right starting mid to late 20's (maybe Sheppard more late 20's). There is a bit of a problem with the protagonist being older because they develop so much in the games and achieve so much. If they were already 40+ you would have to resolve the question why this great hero starts out so unknown and relatively weak. Of course it could be done with good writing.
Hawke and Shepard both had set ages though, while the Warden was open to interpretation. Personally I prefer 25-35 for my protagonists, which incidently is also the age of Hawke and Shepard.
They circumvented the lack of player knowledge with both Hawke and Shepard through clever use of dialogue, which is one of the reason the voiced protagonist is one of the ebst things to ever happen to DA. An example: In the prologue of Mass Effect, you come across a prothean artifact. In the briefing of the mission the player get the dialogue option "Prothean?", which results in Shepard asking "Didn't the Prothean disappear 50,000 years ago?", which implies Shepards previous knowledge of the subject, but initiates convenient plot exposistion for the player audience.