[{"test":1,"arr":[{age:24,name:"a"},{age:55,name:"b"},{age:12,name:"c"},{age:14,name:"d"},{age:67,name:"e"}]}]

db.collection.find({"arr.age":{$lt:30}},{"test":1,"arr":{$filter:{input:"$arr",cond:{$lt:["$$this.age",30]}}}})