Vary validation rule of a field based on the value of another.

I am creating a preschool registration form. It allows a family to enter family information and the uses a repeater area to capture info for each child.

For each child, the parents identify the age (1-5). The age determines the validation of the next question, which asks how many days will the child attend. The minimum number of days varies by age.

How can I create a validation rule for the minimum number of days based on the age?

Thanks!