[{"_id":ObjectId("608be7c608c7de2367c89638"),"status":true,"gender":"Male","first_name":"Vinter","last_name":"R","dob":"1-2-1999"},{"_id":ObjectId("608be7c608c7de2267c89639"),"status":true,"gender":"Male","first_name":"Ray","last_name":"Morgan","dob":"1-2-2015"},{"_id":ObjectId("608be7c608c7de2367c89640"),"status":true,"gender":"Female","first_name":"Lisa","last_name":"K","dob":"1-2-2010"},{"_id":ObjectId("608be7c608c7de2367c89641"),"status":true,"gender":"Male","first_name":"Messi","last_name":"L","dob":"1-2-2005"}]

db.collection.aggregate([{"$addFields":{"currentYear":{$year:new Date()}}},{"$addFields":{"dobYear":{$toInt:{$substr:["$dob",{$subtract:[{$strLenCP:"$dob"},4]},4]}}}},{"$group":{"_id":"$_id","0-18":{"$subtract":["$currentYear","$dobYear"]}}}])