Corns can start breeding at the age of 2 years, but for health sake, most people suggest an age of 3 years and a weight of 300 grams.
You just opened a can of worms with your second question, but I will answer. Corns are known to be cannabilistic, so it really isn't a good idea to keep them together. You can also spread desiese this way, and it will be hard to know who has shed and deficated. But, if you aren't keeping track of all that, and are willing to take the cannablism risk, then it is totally your call.
Corns usually breed early in the year around Feb/march time frame, and lay eggs about a month to 2 months later. Some have been knows to double clutch, which means lay 2 clutches a year.
I breed corns because I like to see what color combinations I can get by mixing and matching what I have, and I enjoy watching them hatch. It also provides a small way to get some money for the food that the snakes will eat each year.