If you do not renew your passport for 15 years, you cannot use renewal form DS-82 and need to use DS-11 (make a personal appearance). For DS-11, a US passport expired for any length of time is acceptable a proof of US citizenship.
Adults whose previous passport was issued when they were children need DS-11, this situation pertains to the poster.
This is all great information but too much of it is contradictory. And, sadly, too much comes from 2007-2010 which may mean it's outdated. I tried using the link provided way up top but it seems to be a dead link. When you go to the government page you get sent in six different directions with none being what you need. I even tried to find contact info on the gov page and, typical government stuff, none of it applied. How hard is it to provide a phone number and/or email address?