[{"_id":ObjectId("6213baa5a013b7c5f1232e23"),"birthDate":ISODate("1973-05-01T13:30:15Z"),"surname":"Johnson","name":"Emma","registerDate":ISODate("1900-06-11T17:30:15Z"),"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("2019-09-14T12:30:15Z")}],"city":"Houston"}]

db.collection.aggregate([{"$addFields":{"ages":{"$map":{"input":"$children","as":"child","in":{$dateDiff:{startDate:"$$child.registerDate",endDate:"$$NOW",unit:"year"}}}}}},{"$addFields":{averageAge:{"$avg":{"$concatArrays":["$ages",[{$dateDiff:{startDate:"$registerDate",endDate:"$$NOW",unit:"year"}}]]}}}}])