id use both vince. i tried to learn french from yr 7 (about 11) and yeah. you know i suck lol.
from my experience, the younger they are the easier it is to understand another language. a friend of mine sofia's first language is swedish, (yay sweden lol) and she came to england and learnt english from the age of about 7. she picked it up real well.
theres also something to do with the whole... sounds thing. diff languages use diff sounds to make the letters and words. after about 8 yrs old, its very hard for a kid to pick up new ones. but its easy for anyone under.
if they learn french sounds as well as english, it will be easier for them to learn french.
so yeah