It takes 15-20 years for them to be fully grown.... its quite a variable, it all depends on growth rate, etc.. there is no set age.
:If i remember correctly I think boxies are full grown at the age of 5-7? Someone correct me if I am wrong, but I am pretty sure thats it. What steve said about determining the age of a turtle is very correct, mine was just an estimate. Not a sure fire age.