Encontre o elemento máximo dentro de uma matriz

db.t63.aggregate([
    {$addFields : {array : {$reduce : {
        input : "$array", 
        initialValue : {age : 0}, 
        in : {$cond: [{$gte : ["$$this.age", "$$value.age"]},"$$this", "$$value"]}}
    }}}
])

{ "_id" : "notImportant", "array" : { "name" : "Carl", "age" : 21 } }