It is impossible to know, since he is already dead at the time of The Wind Waker. He is a kokiri, so his physical age is about 10 years old, which is Link's age in OoT. The kokiri don't grow, so his age measured in years of existence is unknown. Also, including his years spent as a ghost will make the number larger.
Child Link is 9 in OoT, as stated in an interview.