[{users:[{name:"frank",age:15,email:"frank@email.com"},{name:"daniel",age:18,email:"daniel@email.com"},{name:"george",age:18,email:"george@email.com"}]},{users:[{name:"dan",age:19,email:"dan@email.com"},{name:"steve",age:18,email:"steve@email.com"},{name:"chris",age:21,email:"chris@email.com"}]}]

db.collection.aggregate([{"$match":{"$expr":{"$eq":[{"$size":{"$filter":{"input":"$users","cond":{"$lt":["$$this.age",18]}}}},0]}}}])