That's impossible.
We know that Ky became Commander of the Order when he was 16 years old. The War ended in 2175. And, according to the Drama CD, Ky wasn't yet Commander at the Battle of Rome (2173), he was still just a novice Captain.
So that means that Ky couldn't have been older than 16 in 2173, and was at least 16 in 2175.
The first game is set in 2180, so Ky must have been 21 at the youngest and 23 at the oldest. In GGX, he must be 22-24.
If Ky were 25 in the first game or in GGX, that would make him 18-17 at the Battle of Rome. If he got promoted to Commander at 16, how could he still be a Captain at 18-17?
Personally, I think Ky is 22 in GGX/XX/AC+. Because both Roby-Ky and Johnny refer to him as "kid", because Ky specifically points out that he is just a novice Captain at the Battle of Rome (so it would most likely take him a while to raise in the ranks), and because it seems to me like Ky's original profile in the first game implied that he led the Knights to victory shortly after he became Commander. In any case, he can't be any older than 24.
The manga does state that Ky met Kliff Undersn for the first time "15 years ago", but it doesn't say anywhere that Ky was ten at the time.
I've seen the "Ky met Kliff for the first time when he was 10 years old" line in many fan-made profiles, like Wikipedia and TVTropes, but I've never seen it mentioned anywhere in canon. It's not in the novels, not in the manga, not in the Drama CDs, and not in the official profiles of the games. I think it's just a fan theory that got repeated enough times that fans started believing it as canon.
As for the "is Ky gay or straight" argument, I don't see why he should be either gay or straight. Two words: Kinsey Scale. Human sexuality is much, much more complex than "100% homosexual or 100% heterosexual".
