I think this was mainly for Rin and Len's second(?) birthday. Since, you know, adding onto being 14 at release they would now be 16. The only other character I can see Caffein doing this for would be Miku, as the rest of the Japanese Vocaloids are adults or not even close to an adult.
...to think of it, the Engloids are pretty much all adults too, save for Sonika. -shrug-