Both sentences are correct. If you are now an adult, whichever sentences you use, it's right. As a teenager, I was painfully shy Because you use "was", people understand that you are describing the moment you were a teenager. When I was a teenager, I was painfully shy This is also correct. You were also describing the moment or period when you were a teenager. The small difference is probably this: "As" is a preposition, so after "as" you use a noun. But "when" is a relative pronoun, which means you use a clause (a complete sentence with subject verb and so on) Considering this, both of your sentences are grammaticaly correct 😁

Both mean the same thing but the connection is different. When I was a teenager is talking about the time period that you were a teenager and what it was like for you with being shy during your teenage years. And with the other one as a teenager is talking about being a teenager. And what it was like as a teenager with teenage things going on in your life and being shy.