same age

^While that's true, they could be a few months apart.
I think Diana is older. She has the personality of an older sister, but she could be younger than Cassie.

I think they are the same age. There moms had them at the same time. it sounded like they were the same age.

Same age

In the books, at least, it's explained that the other Circle members are all about half a year older than Cassie, with their birthdays all within a month. There's no Jake in the books.