[{"_id":ObjectId("6213baa5a013b7c5f1232e23"),"birthDate":ISODate("1973-05-01T13:30:15Z"),"surname":"Johnson","name":"Emma","registerDate":ISODate("2017-06-11T17:30:15Z"),"children":[{"birthDate":ISODate("1993-05-21T16:30:15Z"),"surname":"Johnson","name":"Liam"},{"birthDate":ISODate("1994-01-21T15:30:15Z"),"surname":"Johnson","name":"Olivia","registerDate":ISODate("2021-09-14T12:30:15Z")}],"city":"Houston"}]

db.collection.aggregate([{$unwind:"$children"},{$addFields:{"children.registrationAge":{$dateDiff:{startDate:"$children.registerDate",endDate:"$$NOW",unit:"year"}}}},{$addFields:{registrationAge:{$dateDiff:{startDate:"$registerDate",endDate:"$$NOW",unit:"year"}}}}])