It really depends on the gecko and the size they will eventually reach. As a general rule of thumb, its not a good idea to breed females before they reach 60g in weight. Laying eggs takes a lot out of them and without the extra reserve they can really hurt themselves. Its also a good idea to wait until they are at least 9 months old, preferably 12 months before breeding even thought sexual maturity is more dependent on size than age. If you have a gecko that will get bigger than 80g it's probably a good idea to wait a bit longer or if you have one of the sub-species that won't reach 60g ever they can be a bit smaller. Males should be similar in size to females.