Based on what I've read, what goes on behind closed doors is of nobody's concern. As sentient beings, they can choose their consent as a free creature. Slaves, OTOH, are bound to their masters. Much depends on the society.
Deity in training, Touya Mochizuki, while he is engaged to several women where harems are common, still, stood by, and encouraged his one friend to confess his love for a beastkin, and, eventually, they married. As for age, again, it depends on the society. We often look down our noses at marriages where the bride has yet to reach puberty, but little more than a century ago, many states had an age of consent for females as young as 10. Not just southern, "redneck" states, either, but the liberal California in it's early statehood. With catlike intensity that starts early, I would assume catgirls may be ready to play the beasts with two backs much earlier that their human counterparts: Especially if you spay them.
Depending on the locale, the species can freely intermingle, but due to various genetic differences, human/beastkin copulation does not always result in a mixed species. If inter-species mating resulted in offspring, there would always be pluses and minuses. A single pregnancy can produce one to ten offspring, which can add a drain on the household budget. One plus, though, is that the youngsters are more easily potty trained: a box of sand in the corner of their room can replace thousands of diapers a year in an average household.(I've seen kids still wearing diapers up through the age of 4, and not just pull ups)
Even without the sexual aspect of having a catgirl, there are other good points. While they involve physical interaction, it would not be considered lewd within our own culture. If you've ever had a cat lay on your lap while watching TV, you see it has a calming effect on you. Both of my cats were cuddled up near my head this morning when I woke up with a feeling of peace. Brushing their fur is not only enjoyable for them, but also relaxing for their human. If you are like me, and carry most of your stress in your shoulders, cats seem to instinctively recognize that and, without a prompt, will start kneading those sore muscles. We won't even start on the effect of their purring to humans.
Yes, we need to drop this war on drugs and put that money into something that will make america great again: the creation of catgirls!