Khan's followers in the movie look a lot older than 15 to me...
They are "hyper-aged", much like how Klingon kids grow to maturity so quickly (ie. Alexander in TNG and DS9).
Too bad about V. But I preferred Judson Scott as Bennu in "Phoenix." (that was a Really Short Series!)
Lt James was in the last six or so episodes of "V: The Series" and, as mentioned, was slated to become a more important character if they'd been renewed for Season Two.
As for "The Phoenix", that's how come Judson Scott has no credit in ST II. His agent pushed for a credit in the opening titles, because "The Phoenix" episodes had yet to air but were anticipated to make Judson a big name. Eventually, Judson was encouraged to "waive billing", which he assumed meant they'd lost the battle for opening credits positioning - but he was horrified, on opening night, to realize that it meant no credit at all.