Well, when you compare Sango to, say, the Sailor Senshi (some of whom are supposed to
fourteen if I remember correctly??), I'd say she could pass for a sixteen-year-old. She's a little well-developed, but it's nothing outrageous.
And I think Miroku can easily pass for 19... I've met a bunch of guys in college who I would have sworn were in their early twenties at the least, but turned out to still be 18 or 19. And even though Miroku tends to act older than he really is, those few times we actually get to see him more open and unguarded, it's obvious he's still pretty young.
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In some episode or other, Miroku is accused of lying about his age, which would seem to me that he's a bit older- perhaps in his later twenties trying to appear younger.
Maybe he was lying to appear older?

I doubt many village headmen would be that willing to accept the help of a monk not even out of his teens. I wish I could recall which episode that was, though.
Doesn't Miroku say in episode 101 that his encounter with that snow woman happened about seven years ago? And he looks around 12 or so in those flashbacks, at least to me.
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Are we referring to what we see in the anime or the manga?
In the manga the whole gang looks a lot younger,
and in the anime they get a little more grown up look.
Even Kikyou and Inuyasha looks childish in the manga,
but in the anime they get this serious adult look (at times...).
That's a good point. The anime tends to give all the characters a more mature look, which makes it even harder to judge their age. And it's really hard to tell by their voices, too. In the dub, Inuyasha sounds older and Miroku sounds younger. It's the opposite in the Japanese version. It can get pretty confusing.