lol Andalite age is one of my favorite topics.
We should get the years right first: THBC started in 1966, ended in 1969. The Andalite Chronicle started in 1974, and "ended" in 1982. The Animorphs regular series started in 1996, and ended, with The One and kamikaze death scene, in 2002.
So really, the whole war, from beginning to end, is less than 40 years long.
No, I'm with Think Again, it's really hard to determine an Andalite's life expectancy. We get a couple of hints in #8:
"If I live 200 years, I'll probably still be known as Elfangor's little brother." 2
So this is an exaggeration in the sense of high-estimate. Living 200 years seems like an accomplishment, but if that's so, then what is normal? 190? Or still like, 70?
Later, Lirem-Arrepoth-Terrouss, the "Head of the Council" (president? general? idk) says to Ax: <Prince Seerow. Yes. He was my first prince. Did you know that? Many centuries ago when I was an aristh like you.> 111
This presents a number of problems.
First of all, it really seems to blatantly contradicted what was said earlier in the same book. I'm pretty hard on inconsistencies in this series, of which there is no shortage, but an inconsistency within the same book almost makes me feel like I'm missing something.
Second of all, "century," by definition is 100 years. What a "year" is, of course, depends on what planet you're measuring from. So maybe an Andalite year is like, two weeks.
Except KAA answered in some Q&A that an Andalite year is 84 months. So that, if anything, makes him at least 700 years old. Yikes. Not really working, especially if he was SEEROW's prince, and we already know Seerow had young children (Aldrea described herself as very young, and her brother was younger than her iirc) in 1966. Either this is like his seventeenth wife, or...KA has no better idea how long Andalites live than we do.
As for your specific question, Alloran was just a warrior in the beginning of the Hork-Bajir Chronicles. It sounded like he took over Seerow's command after he relieved him of duty, which makes me think he was a youngish adult, like you said, 27-32. We'll say 30. In that case, he was 35 in TAC, and like, 57 in the beginning of the regular series. Again, all of these ages are really analogous--maybe Andalites are still "children" well into their forties, and don't become "adults" until their fifties. Who's to say? I think "age" as far as labels is a cultural thing. People were mothers and fathers, getting married and starting careers in their teens back in the middle ages, and now they're waiting much longer to do all of that. I kind of think cultures tend to wait longer to "grow up" the more advanced they get, the longer their life expectancies. Hell, Ax might even be in his thirties. KAA did say he was "three or four," which makes him 21 to 28 human years old.
WHO KNOWS.