Alumni Interview

I thanked the interviewer verbally but did not send an email to him. How much does this matter

I would send that email today. I doubt it made a difference in your chances but it’s just good manners.

how long ago was the interview?

It’s always a good idea to send a thank you note by the end of the same day you had an interview. The interviewer might give you a pass because you’re a teenager, but you never know.