I tend to think he is between 8-14. I have a friend who is almost 15 who you could almost mistake him for being a 12 year old.
Toon Link is definitely at that age where he isn't a really a child but he isn't quite an adult. In Wind Waker he may be older yet depending on the presumed age of the hero of time that they use to know when to have the rite of passage on Outset Island. He is definitely under 18 though.
I think it was his 12th birthday at the beginning of Wind Waker.
He was probably still 12 in Phantom Hourglass
and in Spirit Tracks which took place 100 years later..I think that counterpart was 12 also.