First things first, cats have their first heat cycle around 6 months of age. Not all females will exhibit the signs that most females cats do such as vocalizing, positioning, teasing and such. If you are not absolutely certain with no doubt whatsoever...separate the two of them now. A 6 month old feline mother is relatively the same as a 14 year old human mother. If the capability is there and fate makes a way, reproduction is possible. Beyond that, it's damaging to the body of a growing young one and it can sap their bones and organs of vital nutrients...just to support the offspring. The damages it can have on their health can be life-threatening. I presume you know this...because you've got a good handle on what an appropriate age is for breeding: 1.5 years.
Assuming she is not in heat, there's a chance that another cat may be in heat close by. Even though its not YOUR female, he may be assuming that it is and assuming his duty as an intact male. He CAN hurt your female if this behavior is permitted to continue. Males who are rough can damage the external and internal genitalia. Even worse, continued attempts from the male while the female is not receptive can make a normally sweet and affectionate house cat turn into a scared one that refuses to be present around the male and may even become aggressive towards him or the other family members (especially when she's in heat). Until she is ready to breed (and you're ready for her to do the same), make sure this is not taking place.
