When we brought our most recent kitten home, he was totally infested with fleas. We'd never had a flea problem with any of our cats, we weren't sure what to do for a cat that was only about 6 weeks old, so we went to the pet store and found Hartz (among several other similar brands) and they all said not to use on kittens under 12 weeks. So we didn't buy them.
One of my co-workers volunteers at an emergency animal hospital, and even though she's not a vet, I figured I'd pick her brain and ask her what the dangers would be of using it on a kitten younger than 12 weeks, and she freaked out when I asked her! She told me not only not to use it on a kitten under 12 weeks, but not to ever use those on any cat of any age. She said it's unbelievable how often they get cats in the emergency room, in shock, after having used those products, and sometimes, unfortunately, they can't save them.
Chuckles is now about 3 months, and still has a flea or two, but it's way under control (thanks to the motherly-grooming skills of my other cat, Raven). We're probably gonna get some Advantage or Frontline within the next week.