My daughter has US citizenship as well as citizenship with an EU country. We live in the EU. She's under 16. When her passport expires in a couple of years in February, she'll still be under 16 until November that year.
The rules for renewal-by-mail of a U.S. passport require that the passport holder have been 16 when the expired/expiring passport was issued.
So I'm inclined to wait to renew until she turns 16. I know we won't be able to renew this time by mail anyway, but if we renew before expiration, we also won't be able to renew by mail next time, whereas if we wait till after her birthday, then next time she can renew by mail.
Obviously if the passport is expired, she won't be able to travel to the US. Her residency in Europe is based on her EU citizenship, so that won't be affected.
Other than not travelling to the US, are there any other consequences we should be concerned about if we let her passport expire and only renew it later months later when she's 16?