A lot oft people fleeing other countries don't carry documentation and, especially young refugees, often don't even know their birthday
I know 3 kids from Syria who just have no idea what their birthday is because they were never told. There's no documentation available (anymore) and you can't ASK the government of the country they were fleeing from what their birthday is for obvious reasons. So a doctor checks him and they make their best guess at their year oft birthday. They day is obviously irrelevant so they go with 1/1.
The point of asylum isn't to strengten your workforce or improve your economy. It's a side effect. You can't, by international laws, deny a refugee asylum based in their (lack of) knowledge. You can't send someone back to an unsafe country. It's just literally not allowed, for good reason.
In a lot of cases that is simply not true. Having spoken to actual police and a dentist who does some sort of dental age testing procedure, a lot of them fake their age so they are underage and thus do not get higher sentences if convicted. I also know second-hand(from my fathers' partner who's a middle school teacher) that there are even a lot of cases in schools where you have boys sitting in the class which are obviously 18+ but are officially 14 in their documentation (and again born on 1.1.) and you basically can't do anything unless they become criminal and get their age tested.