YSK: How to document that you are a US citizen
https://www.usa.gov/prove-us-citizenship
The most common way to show that you are a US citizen is to show a passport. However, only about 50% of US citizens have a passport.
An alternative to a passport is a Certificate of Citizenship.
Certificates of Citizenship and Naturalization show proof that someone is a U.S. citizen. The website https://www.usa.gov/prove-us-citizenship shows how you can get or replace these documents.
Prove your citizenship: born in the U.S. with no birth certificate. If you were born in the U.S. and have no birth certificate, learn how to get documentation to prove you are a U.S. citizen.
Prove your citizenship: born outside the U.S. to a U.S. citizen parent. Prove your U.S. citizenship without a birth certificate if you were born outside the U.S. to a U.S. citizen.
Elsewhere in a comment someone mentioned Passport Cards. iirc they’re only valid for travel to Mexico and Canada, but they are proofs of citizenship, and you can have a card as well as a passport.
Its the passport that also works for domestic air travel now that they banned drivers’ licenses.
It does not have your address on it, so its a safer document to use to prove your age.
* banned drivers licenses that don’t meet the requirements that have had their implementation date pushed back repeatedly for a decade+. We will do anything and everything but have an actual national ID.
I don’t think any drivers licenses count as proof of citizenship though. (Even before, as noted, factoring in that they’re not looking.)