Kitten on adult food?

[InfiniteMystery]

Hi Guys, just looking for a little input here....

We have 3 kitties - a 7 year old(longish hair, gets the occaisional hairball), a 3 year old (who is overweight), and an unneutered kitten (approximately 3-4 months, is healthy, good weight).

Now my dilemma.... I would ideally like to keep the kitten on kitten food until he's almost a year, but seeing as no one ever wants to eat their own food (and I can't always stand around to "monitor") the overweight kitty inevitably ends up eating the kitten food. Seeing as the kitten is a healthy weight, I'm more concerned about what the overweight kitty is eating. So my question - is it okay for a kitten to be on adult food at 3-4 months old? Could I possibly effect his long term health by taking him off of kitten food so early? I would ideally like to feed them all the same food (the 2 adults are on Royal Canin Indoor formula).

Any ideas or suggestions would be greatly appreciated! Thanks!