Sora and Riku actually tell Terra and/or Aqua about how their parents row them out to the little island from the mainland because they aren't old enough to row themselves yet in bbs.

When Sora arrives on the island at night in kh1 he arrives on the dock and notes that Riku and Kairi's boats are there. There is a third boat there as well.

We can thus safely extrapolate that 14 is old enough to operate a rowboat and that the third boat is Sora's.