It's hard to say really. There are some breeds where cats keep growing until they are 3-4 years old (at a slower rate obviously
).
If you spayed them at an early age they are more likely to keep growing. As a rule early spaying/neutering makes for larger cats (And I have 2 giants to testify for it!)
Anne is right. If they have a particular breed mixed in there they could grow until they are 4 or so. I have a Maine Coon mix who grew till he was 3 and now is at a steady 22lbs(he was neutered at 7 months). My other girl who is also a mix is still growing and she is a year. I found that my others usually hit their peaks at about 1.5 years.