[{_id:"notImportant",array:[{name:"Peter",age:17},{name:"Carl",age:21},{name:"Ben",age:15}]}]

db.collection.aggregate([{"$match":{_id:"notImportant"}},{"$unwind":"$array"},{"$setWindowFields":{"sortBy":{"array.age":-1},"output":{"rank":{$rank:{}}}}},{"$match":{rank:1}},{"$unset":"rank"}])