All three of the kitties you see in my sig got quite a bit of KMR when they were young. Sinbad was a half-Himalayan kitty who for some reason was the runt of the litter, 2/3 the size of her littermates, when I adopted her when she was about six weeks old. I gave her KMR for at least a month after that, probably closer to two months, and she went from a tiny kitten to being nearly a yard long and weighing close to 15 lbs most of her adult life. Tiger was a thin feral kitten about six weeks old and needed the extra nutrition. Frosty was abandoned by a bike trail when she was about ten days old and got round the clock feedings of KMR. I'd guess they all had some KMR till they were at least ten weeks old, maybe 12 weeks, and I wouldn't hesitate to give it to an older kitten. It's been years since I looked at a can, but my recollection was that it was also recommended for sick and nursing cats, so I don't think it could hurt an undersized kitten.
Here's a web page I just found on a cat who's been on a liquid diet for life:http://www.theanimalrescue.com/bazil.html
Apparently it's KMR he's on, since his owner is in a program the company has and sends them UPCs to get free formula. That kitty is five years old now.