From a breeder's point of view, I am sorry Lito - no offence intended - but this is just about the worst suggestion I ever saw ;-)
6 months to grow up? That would be the end of KittyCatS breeding, markets, selling, auctions etc. just like that.
However, if I understand you right, you basically want the option to keep your cats for as long as you like, and then turn on their breeding at whatever time you feel is right... And that could be solved by giving them so-and-so many breedings instead of a certain age. For instance - when you birth a cat, it can breed 10 times total, no matter the age. So, if you breed it every 7 days, it's breeding would be over and done at the age of 77 (Since it takes 14 days to get first kitten - and remember, this is just an example.). -But if you never breed it, you could turn on it's breeding when it was 300 days old if you liked - and THEN start to have kittens.
While most would think that breeding on old cats like that would be somewhat silly, since it obviously doesn't carry any of the new traits that are often the most desirable, this model is the only one I see that could solve this particular problem. However, I am afraid it wouldn't be received all that well by most breeders...
Saga