Database
[{citizens:[{name:"John",birthday:"1993/07/13"},{name:"Sarah",birthday:"1996/07/13"},{name:"Natalia",birthday:"2015/07/13"}]},{citizens:[{name:"Leo",birthday:"1994/02/08"},{name:"Paul",birthday:"1934/09/13"},{name:"Rego",birthday:"2019/01/29"}]}]
Query
db.collection.aggregate([{$set:{citizens:{$map:{input:"$citizens",in:{$mergeObjects:[{status:{$cond:[{$lt:[{$toDate:"$$this.birthday"},ISODate("2004-07-27T00:00:00.000Z")]},"adult","$$REMOVE"]}},"$$this"]}}}}},{$merge:{into:"collection"}}])