Student Loans don't automatically expire after you retire nor do the student loans automatically go away after 20 to 25 years.
To be eligible for student loan forgiveness you have to be enrolled in a payment plan with the program known as Pay as you earn repayment plan or PAYE plan.
As long as you're continuing to make payments over those 20 to 25 years then your student loans can be forgiven after 20 to 25 years have passed depending on the student loan.
Some retirees have reported that they've had their social security benefits reduced to pay for student loans so you do still need to pay on your student loans even in retirement unless they're forgiven through a student loan forgiveness program.