Database
[{"state":"NY","DOB":"2000-01-02"},{"state":"NY","DOB":"2002/03/04"},{"state":"NY","DOB":"00-00-00"},{"state":"NY","DOB":"male"},]
Query
db.collection.aggregate([{$set:{Age:{$let:{vars:{dob:{$dateFromString:{dateString:"$DOB",onError:"None"}}},in:{$cond:[{$eq:["$$dob","None"]},"None",{$round:{$divide:[{$subtract:["$$NOW","$$dob"]},31536000000]}}]}}}}}])