The 6 year difference between 18 and 24 may seem like a lot, but at that age, it's not really a big deal, when it comes to girls fighting. Of course, if the 24 year old has more fight experience and perhaps some training, that would make a difference, but just their ages, I don't know that it would matter. Both women would be about the same size at that age, excepting for things like weight, body type, etc.
I think a 6 year difference would definitely be a big deal in younger girls, but not at that age.