KotOR takes place almost 20 years or so after the Mandalorian Wars, right? So by the time of KotOR2, Revan must be, what, 40something?
The Mandalorian wars began in 3,976 BBY(20 years before KotOR) but ended in 3,960 BBY(4 years before KotOR). However, Revan didn't enter the war until 3,963 BBY(7 years before KotOR). Revan's age is up to the each individual player, so besides being about at least 25, Revan is however old you say he is.
As LI says, there's no canon age, nor - I suspect - will there ever be. We can assume with near-certainty that how old he thinks he is as "mind-wipe Revan" is incorrect, however, considering that "How old are you?" is one of the questions that Bastila asks Revan to confirm that the Council's conditioning is still in place. I, personally, place Revan (as opposed to the false identity) at 29. Young enough to be "young" and look like the "kid" he is often referred to as throughout KotOR, but old enough to have been an experienced and well known Jedi Knight by the time the Mandalorians invade. In the KotOR comics, therefore, he would currently be twenty-three - Anakin's age in RotS. That would also places his DOB somewhere around 3985 BBY, which works with my pet theory that he is the love child of Nomi Sunrider and Ulic Qel-Droma, concieved on Rhen Var towards the end of 3986 BBY.
I don't really want Revan and Bastila to have a huge age difference. According to Wookieepedia, SHE is 23 at the time of the first game. Personally, I'd put Revan at about 25 or 26. That would have made him about 17 or 18 when he went off to fight in the Mandalorian Wars. That's not much different from Anakin, who was only 19 when the Clone Wars started. But that's just me.
Anakin was 20 when the clone wars started. He wasn't responsible for leading a huge breakaway section of the Jedi either. I can see Revan being knighted at 17/18... I can't see him being the charismatic and mysterious "Revanchist Leader" at 17/18. The dude is clearly not a teenager. Bastila's age has not been confirmed. BioWare themselves stated a couple of years back that they intended for her to be twenty, but I think they also acknowledged that their intention wasn't canon. (She's "not that much older than Mission", remember?) Insofar as "Mindwipe Revan" goes, I would say his age was about 25/26. His real age, however... doesn't make a great deal of sense to me if he's that young in KotOR I.
Sounds about right to me, as well. All of the in-game faces seemed to be in the mid-twenties range to me, if not younger. Revan might've gone to war at 16 ... although, if that's the case, Malak would probably be older. As for Bastila being 23 ... she looks younger than that, but not beyond the realm of possibility. That'd put 9 years between her and Mission, though(IIRC, Mission was definitely 14 in KotOR I), which'd stretch some dialog a bit (unless Mission was in error about Bastila's age). Unless the effects of the Force upon aging are noticeable in early adulthood ...