Well I'm 50, and I hope to keep going for a while yet..... But anyway, if you're refering to dogs. I wouldn't breed a female past 8. Would prefer no later than 6.
Also, do you really need to breed her? Once a female has pups, you increase here chances of getting mammaliarian cancer 200x. And even if you get her spayed after breeding this will not reduce the risk once she has had a litter.