Nested output buckets' aggregation

Elastic Stack Elasticsearch
1

Here is my doc type: vehicles is a nested type.

document1 -

{
  "customerAge": 30,
  "vehicles": [
    {
      "brand": "Altima",
      "year": 2005
    },
    {
      "brand": "Corolla",
      "year": 2006
    },
    {
      "brand": "Camry",
      "year": 2005
    }
  ]
}

document2 -

{
  "customerAge": 50,
  "vehicles": [
    {
      "brand": "Altima",
      "year": 2009
    },
    {
      "brand": "Corolla",
      "year": 2010
    }
  ]
}

I am trying to create buckets of vehicle brand and calculate the averages of customer age. So in this example "Altima" bucket's average customer age will be 40.

I tried this query and the averages are null.

Could someone please help what am I doing incorrectly?

{
   "size": 0,
   "aggs": {
      "nestedtop": {
         "nested": {
            "path": "vehicles"
         },
         "aggs": {
            "testagg": {
               "terms": {
                  "field": "vehicles.brand"
               },
               "aggs": {
                  "filterx3939d": {
                     "filter": {
                        "match_all": {}
                     },
                     "aggs": {
                        "filterx3939d": {
                           "min": {
                              "field": "customerAge"
                           }
                        }
                     }
                  }
               }
            }
         }
      }
   }
}