It's going to depend on the snake. I know some buy into the 3,3,3 guideline. I don't buy it. I believe that following that is unsafe. A corn snakes age has little to nothing to do with breeding readiness. They aren't built like mammals and that's where we get that association from.
You need to learn how to evaluate each snake, individually, to determine breeding readiness. That's not saying that a snake is ready at 1.5 years. That's saying that with a good feeding plan(not power feeding) and a good handling/exercise plan, a corn can be ready at as young as a year and a half, two years, two and a half years, etc.
As I mentioned, I don't buy the 3,3,3 guideline at all. That is a generic grouping. All that guideline means is that you have a 3 year old, 300 gram and 3 foot snake. That doesn't mean that you have an animal that is ready to breed.
What you need to do is look at your snakes feeding schedule. How much has she eaten? How often?
Then look at the muscle tone of the snake. Is she over or under weight for her length? Does she have good muscle tone? Does she have a rounded belly or a nice strong belly? Does she have a little fat stores, because she is going to lose weight when ultimately lays?
What is her length? Is it reasonably proportional to her weight?
These are important questions you need to answer. If you follow a generic guideline and breed as soon as you meet the minimum requirement of the guideline, then you could be setting yourself up for failure. Meaning jeopardizing the life of the snake. A 2.5 year old, 250 gram, 30 inch snake may be better built for breeding then a 3,3,3 snake. AND vice versa! There isn't a set rule or guideline. Just an educated guess that is made by looking at each snake individually.
This method isn't the same from males to females. Males can breed much younger than females. The only risk with males is if they go on a prolonged hunger strike.
Wayne