Unless its displayed somewhere where another person can find it, you are screwed.
I did exactly the same. I suspect I was either not bothering when I joined or deliberately put an incorrect date in as I could see no reason why it was needed. Of course I know now but it didn’t say at the time.
I now can’t log in as I need to give my DoB
FB help is non existent and every way I tried, even though the imaginary help channels wanted a DoB.
Grrrr
Any way I just registered again, (same name, even same pic)contacted my contacts and said delete the old one and Spam any messages to do with the old account. I suppose if you were some saddo with a million so called friends it might be a pain but I bet there are only a few who matter.
If anyone has a sure fire way of getting to the old account it may be handy. doubt it though.