How do I prevent a 3 year old from enrolling in a class that is for 5 year olds? (or a boy from enrolling into a girls only class)?

Use the Age and Gender filter settings on the Web Reg Settings page. This is covered in detail in the Online Web Registration Settings page.  Age is calculated as of the Class Cutoff Date.  If no Cutoff Date is listed, the Class Start Date is used. The Class must have both a minimum and maximum age (in years and months) entered for the system to correctly calculate age eligibility for online enrollments.