[{name:"AAA",age:14,country:"us"},{name:"BBB",age:13,country:"us"},{name:"CCC",age:12,country:null},{name:"DDD",age:12,country:null}]

db.collection.aggregate([{$group:{_id:"$country",people:{$push:{name:"$name",age:"$age"}},age:{$addToSet:"$age"}}},{$project:{_id:0,country:"$_id",people:1,age:{"$cond":{"if":{"$eq":[{$size:"$age"},1]},"then":{$arrayElemAt:["$age",0]},"else":0}}}}])