Lol, when I first discovered KAITO, I looked him up on the vocaloid wiki and it сказал(-а) he has no certain age, but is drawn by Kei (an official vocaloid illustrator) as a middle aged man. I was like "What the heck?!!!" But yeah, he's a very young, immature young adult. Probably in his early 20s. Lol
there! Kaito has no official age. He was released on the first VOCALOID engine where age, weight, and height where not дана for the first Crypton VOCALOID's (Kaito and Meiko). But, Фаны often estimate him to be 20. The only Crypton VOCALOID's that have official ages, weight, and height are Hatsune Miku, Luka Megurine,and Rin and Len Kagamine. If Ты want to know theirs, just check out the handy VOCALOID wiki.
To me it makes sense to say 18. As of the song, ACUTE, has Kaito, Miku, and Luka, and it is a Любовь song. Luka is 20 and Miku is 16. It would make sense for Kaito to be in between them, sense ACUTE is a Любовь song.