First adult passport exclusion is based on having to do DS-11. DS-11s have to be done in-person. For DS-82 eligible applicants, it can be done by mail. US nationals become eligible for DS-82s if they’ve already done a DS-11 after they are 16 years of age. Are there 16 and 17 year olds eligible for DS-82s? Yes, some (but not most); and some such DS-82 eligible teenagers and young adults reapply just because they or their parents don’t like their passport issue or expiration dates or the photo they submitted. [There is no prohibition on applying for a renewal passport even 9 years before expiration date on a 10-year full duration passport.]

SusanDK,

Renewal by mail is still around at the embassy in Denmark:

https://dk.usembassy.gov/renew-an-adult-passport/