How do I know this? The actor, Robert Rusler, was born in September 1965. He was 29+30 yrs. old in 1995 (Depending if the filming was before and/or after his birthday.) So we don't just have to judge by how old he 'looks'; we actually know how old he actually is in the scenes we're watching.
It's interesting background in terms of how the casting director saw the character, but it doesn't wash at all from an 'in universe' perspective. Even ignoring that we have printed ages for characters which never match those of the actors, the math can never work out. Consider Mark Hamill, who in the course of four years (1994-1997) played Blair four times which covered nearly twenty years (2654-2673) of his life. If he was the same age as Blair for Wing Commander III (which he wasn't) then he certainly couldn't have been for IV, Prophecy or Academy (the real time/story time gaps being 1 year/4 years, 3 years/12 years and 2 years/-14 years, respectively).
1. Blair's DNA was chosen while he was very young, actually not because of his piloting skills, but rather his Pilgrim ancestry
Interesting thought. The indication is that this is the case (Pilgrim heritage or not). The Genetic Enhancement program started twenty years before Wing Commander IV, which would put it around 2653. Blair was chosen as a template source while he was still at the Academy. *Why* he was chosen is unknown -- it's possible it was because of his Pilgrim 'powers' and it's possible that it was because he was a top-performing student.
Both of these things would have been known to the brass at the time. I would caution anyone arguing that the former is a clear retcon to remember that the earliest Wing Commander movie script was working its way around at the same time Wing Commander IV and its novelization came about... so the idea that Blair was more than just a great pilot likely already existed.
There's also a good amount of intrigue suggested by later materials. We know that Tolwyn sent a psychologist to report on Blair throughout his years at the Academy. This could be for any number of reasons -- perhaps because he served with Blair's father in the last war, perhaps he wanted to poach the Academy's top performer... but in Paladin's letter to Tolwyn at the end of the Confederation Handbook there is a reference to Blair's Pilgrim heritage being somehow important (specifically because of who his maternal grandfather was).
To cut things, however interesting, short: this gives us the 'lower bound' for Seether's enhancement - it couldn't have happened before 2653 (since the program didn't exist before then). Of course, the upper bound is still 2673 at this point...
Then we later find out that Hawk flew with Iceman as his mentor, this puts him on or before Iceman's career on the Claw. (Do we know his service record? Is it possible Hawk met him before he was on the claw?)
Good. You are absolutely correct. Placing Seether in the timeline requires that we place Iceman and Hawk, and their relationship.
We know that Hawk began his career, on the Tiger's Claw, as an enlisted man and at an unspecified time. Based on clarifying evidence, it seems pretty clear that "when I first signed on with Confed" means as a pilot rather than as a willing conscript (he saw Seether, after all, at flight school). As a tech he struck up his friendship with Iceman, who helped him transfer to Officer Candidacy School. Hawk left the Tiger's Claw to attend OCS and Flight School, where he apparently saw Seether for the first time. It was also during this time that his homeworld was bombed and he became closer to Iceman. The story goes that, at this time, Seether was a new pilot with an amazing natural talent. This was apparently seen by black ops and he was taken away by the Genetic Enhancement program. Hawk graduates from OCS/Flight School and then goes back to an unspecified ship where he serves (for at least one mission) with Iceman (as related in Prophecy).
So when was this? In short, it had to be between 2653 and 2656 - since it needs to be after the program comes into existence (duh) in 2653 and before Hawk graduates and goes back to fly with Iceman before his death. So by Wing Commander IV, Seether has been flying for between 17 and 20 years.
Which jives with Wing Commander IV in an interesting way -- note from his chest that Seether wear's a Colonel's insignia and the same set of ribbons as Blair (but worn in reverse order). He's not some young upstart... he's Blair's mirror (young as he may look -- remember, he's been enhanced...).