'How to get average of two values which are in different objects?

My data looks like:

[
  {
    "_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"
  }
]

I want to calculate registration ages of persons who have registerDate. In this example, I should get average of registration ages of Emma(parent) and Olivia(child). I tried this. I calculated the registration ages and added them as a field. I got Olivia's registration age. However, I have duplicate registration ages of Emma. I want to use one of them. How can I get average of registration ages which belong to Emma and Olivia. I mean (5+1)/2=3. You can see these values in the link that added.

mongodbaggregation-framework


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