Although he didn't know it at first, Alec's half-faie. Everyone who's known faie figure it out just about the moment they see him, but Alec's a country boy from the north to whom faie are pretty much a ... well, a fairy tale. He was very excited to meet someone who knew faie (Seregil neglected to inform Alec at first that Seregil was faie, also) but skeptical of the fact that faie live for hundreds of years; they're just stories to him at first, before he gets out more into the wide world.
"But they are real?" Alec persisted; this was more than Seregil had let on previously. "Have you ever seen any? What are they like?"
Seregil smiled.. "Not so different from you and me, really."
Anyway, so. Since they live hundreds of years and age extremely slowly, they tend to look younger than they are. Seregil is "not much older than Alec" and "looks like a man of twenty" and is just about sixty years old. By his own people's reckoning, he's still a child, and in Traitor's Moon there's a certain amount of disjoint from the faie who knew him, as they see him acting so grown up from living with humans and watching them age and die.
Alec, however... well, I take Alec from a chronological year before Traitor's Moon, but here's a quick excerpt from when his age comes up there... he's sitting at a table with two kids who look barely into their teens if that. The girl, who looks maybe 14, is showing off.
"And I'm only twenty-two!"
"Twenty-two?" Alec turned to Mial in surprise. "And you?"
The young Aurenfaie grinned. "Thirty. How old are you?"
"Almost nineteen," Alec replied, suddenly feeling a bit strange.
Mial stared at him a moment, then nodded. "It's the same with some of our half-breed cousins; you mature much faster at first. You might want to keep your age to yourself once you get over the mountains, though. The purer clans don't understand [half-breeds] the way we do here. The last thing your [beloved] needs is another scandal."
Alec felt his face go warm. "Thank you. I'll keep that in mind.
Of course, that has to do with how maturity is perceived, not how Alec physically looks. Earlier books, however, shows that! For example, when Alec and Seregil first meet at the start of the first book, Seregil assumes Alec isn't old enough to be out on his own -- and in a pre-industrial society, that's pretty much prepubescent.
"How old are you, anyway? You look young to be roaming around all by yourself."
"Sixteen last summer," Alec replied a bit gruffly. He was often taken for younger than he was.
Now, the thing is... sixteen looks young. If you look distinctly younger than sixteen, enough to surprise someone, it's probably because they look at least two to three years younger. (A sixteen-year-old being mistaken for being fifteen is no big surprise, after all).
Moreover, it is strongly implied that Alec hasn't hit puberty until the second book (he's sixteen over both the first and second book, only 17 when some time is ellipsed at the end of the second). In the first book, he is constantly referred to as boy, and when he, Seregil, and Micum are wanted men, their would-be captors are looking for "two men and a boy". He passes as a noblewoman's squire for a while, which implies he at least looks prepubescent, as a sexually mature boy alone taking care of a noblewoman is a definite no-no. And Seregil himself implies that Alec is pre-pubescent when he first introduces Alec to the concept of whorehouses -- Alec is shocked to find same-sex whorehouses in the district as well as opposite-sex, and Seregil teases him, "By the four, haven't you heard the saying, 'Never spurn the dish untasted'? And here you haven't even had a smell of the kitchen yet!" This seems, at least to me, to imply that Seregil's pointing out that he shouldn't decide what he does and doesn't like to eat if he's never been hungry -- in other words, don't decide his sexual preferences before he's ever felt lust. (Of course, Seregil also seems perfectly content to think that in the right environment, Alec'll learn to feel lust and fast; he suggests they go back to the whorehouses, and soon -- they do, in book two. So Alec is obviously an age that's on the verge of puberty, right between child and teen).
It's definitely in his sixteenth year, I'd say, that Alec hits puberty. Watching a friend breastfeed her baby goes from extremely awkward discomfort for him to a daily pleasure and he definitely starts getting interested in girls as his sixteenth year passes (again, he's sixteen through the majority of both first two books). He's definitely reached sexual maturity by the time he's, um, date-raped by a hot foreign sorceress, and visits a whorehouse now and then (after wacky hijinks where he once followed Seregil into the wrong one). He's still sixteen when he starts to get attracted to Seregil, and he's 17 by the time he's in a committed relationship with him.
So, essentially, the age of hitting puberty/sexual growth is physically at sixteen for Alec, when it's more at 13-14 for most boys; hence, I figure that's probably how much younger he looks than his physical age and why I chose a young teen for a PB.
....I feel like this is somehow more convoluted than I set out to have it be. XD