I was able to find out the month by phoning PayPal and asking. However this may not be an option for those who do not want to raise their head above the parapet.

To be explicit for anyone who gets confused with date-based math: your "joined in" year minus the year of your birth needs to be at least 19 to be safe. If it shows 18, you may have been 17 instead of 18 when you joined.

the best way to find our the exact date is to find the sign up email, if you still have access to it.