It's impossible to tell for sure, but a late twenties estimate may be a bit too young. Revan has been stated to be a prominent and renowned Jedi Knight by the time the Mandalorian Wars began. And from the start of that war to the end of the Jedi Civil War, nine years pass. Obi-Wan for instance, was 25 by the time he was knighted, and only because he defeated Maul. Anakin was ~22, and he skipped the standard trial business, too. Revan might have been teh shiznit and all, but not much younger than that.
On the other hand, the only data we have on Carth is that he is a somewhat competent Republic flyman by the start of the Mandalorian Wars, and that he has a son that's old enough to be a Sith apprentice, nine years later. That could make Carth as young as 36 by the time of KotOR, younger possibly.
But given the inconsistent mess that is the EU and the all too frequent retcons it suffers, Revan and Carth might be any age - logic is irrelevant and facts can be, and often are, retroactively reinvented.
This is probably a WAY nerdy thing to do, but since I happen to have a Prima Games Strategy Guide for KOTOR, I guess I'll go ahead and plagiarise it.
Carth Onasi:
A career soldier at 38, Carth has seen more than his fair share of war. A skilled pilot and superior tactician, he has always faced his duty with grim determination, believinng that loyalty and dedication to the Republic would see him through. He has come to discover, however, that not everyone holds this opinion.
Hailed as a hero of the Mandalorian War, when the Sith began threatening Republic space again, Carth thought that they could be fought like any other enemy, but their influence extends far past simple starships and blasters. Though he will not speak of it, a very personal betrayal has affected him deeply. Duty called him to fight the Sith, but now outright hatred threatens to replace it.
It is no longer clear whether he fights to stop the destruction or avenge what has already been destroyed.
This makes no mention of the PC's age, however.