What practical impact does it have on the story, though? In the books, novices aren't accepted (usually) once they're too old because they're too independent by that point. Letting novices start younger, especially if they've already started channeling, could fit in with that dogma.
Regarding Mat's healing, I heard someone else point out that Moiraine does partially heal Mat in the books without everything being completely healed (TEotW ch. 41-42), and that tracks with what we saw on screen. She does mention something along the lines of him still being in danger (I can't...