You can not apply until you meet all the requirements, including 5 years as a teacher.

Also, the program will only forgive student loans you currently owe.  This means you don't get a refund for any payments made.  If the loans are already paid off in 5 years, then there is no benefit to the program. 

https://studentaid.ed.gov/repay-loans/forgiveness-cancellation/charts/teacher