If he was sixteen, do you not think that he'd be a bit bigger? Also, if he's aged six years since the beginning of the series, so has everyone else...and that makes Brock at least twenty. So why doesn't he have a car yet? Also, Ash doesn't care about girls, so its unlikely that he's a pubescent sixteen-year-old. I understand that the producer's wouldn't want a full-on romance, but he'd be a bit more like Brock, at least. The only time when Ash has shown interest in a girl is when he met Giselle, from the Pokemon Tech episode.
EDIT: just realised this would mean that brock (a twenty year-old) was travelling with a sixteen year-old boy and a ten year-old girl. That's just...weird. :\