How old does a cat have to be to give them a flea bath?

To give them a bath with flea shampoo you should wait (like the label should say) until the kitten is older then 12 weeks. An alternative to flea shampoo would be just to wash your kitten with dawn soap, which you can use before it is 12 weeks. An easy way to do this is to wrap your kitten in a towel (to protect yourself and the kitten) and submerge it (except for the head). Most of the fleas will drown, then use the soap to wash your kitten.