The point here though is that every forum can create its own forum "rules and regulations". The statement itself is not failed and even if you made the DOB field mandatory, then it does not take einstien to work out that anyone can enter a date of birth that is untrue. Example on these forums and ours - some people are over a 100 years old

The point however should be that "due dilligence" is practiced, and if a forum member signs up and gives a DOB that is below your forums acceptable policies, then admin or mods should suspend the account
I did exactly that on one of our forums and sent the user a PM explaining that because their DOB registered was below our acceptable age, then 24 hours later their account would be suspended