Is it wrong to email pictures of yourself in your underwear if you’re a teenager?

There are several ways to look at this.

Legal – If you are under the age of 18, and you take a sexually explicit photo of yourself, you are breaking the law.

The crime of creating child pornography was committed by YOU. There is no age requirement for consent to have sex, nor is there any requirement that you generate explicit content as a result.
The crime of child pornography dissemination has now been committed by YOU, if you transmit it.
Because if they don’t, they’re breaking the law by possessing child pornography, which is a felony.
It may come back to haunt you later.

Once an image is sent, you have no control over what happens to it.
You can become renowned or hold a position of moral authority, and a picture of you doing so would be sought for. Don’t expect your crush to save a picture of you from being sold for $50,000.
A lot of teenagers do this, and it’s typically safe, but it has the potential to be dangerous for others.

If you’re serious about sending it, use caution and follow the law to avoid legal ramifications (you and recepient).