If you were the one doing the teasing back in middle school, own it. Growing up includes going back.

Twelve explains it and does not cancel it. The person on the other end may still remember the specific thing, in more detail than you do, because what was a joke to you was a Tuesday they had to get through. Growing up means the account is now yours, even though the person who opened it was younger.

If you can reach them, a short message without excuses is enough: you remember, you are sorry. If you cannot, the repair is in how you treat the next person who is an easy target. Both are real, and neither requires you to hate your younger self.