Welcome! This forum has a treasure trove of great info – Scouters helping Scouters! Just a heads up, though - all content, information, and opinions shared on this forum are those of the author, not the BSA.
I had duplicate account merged awhile back, but now while I try submit my application to join my child’s troop as an adult, I get the you must be 18 message. I tracked it down to where I think the error is. My birthdate in Profile says invalid, but it is a non editable field. So, even though I have my YPT and several trainings done, I can’t submit the application.
The problem is you have 2 BSA Member IDs (MIDs). The MID that was set as primary on your my.scouting.org does not have your date of birth. I switched the primary MID to the one with your DOB. You should now be able to register.
I also updated the MID on your Scoutbook account to the MID with your DOB. Your training should sync from the former primary MID to this MID within 24 hours.