It depends, I have two corns that are the same age, both fed "right" and one is almost four times the size of the other :shrugs:
It usually takes mine about 10 months to hit two feet (estimating, I haven't measured my corns in a while). I tried probing my petco snow, who had popped as a female when I bought her, and I still believe her to be a female, however, due to the young age and small size, I will be trying again later this year.
I personally will not pop anything older than 6 weeks of age. My friend who is a breeder will pop just about anything, but he is much more experienced. I feel that the risk of popping an older snake is not worth it, I'd rather have them probed. I just got a probing kit last weekend and they are super easy to use (and oh so fascinating!).