[{"items":[{"_id":"Red","15-17":1},{"_id":"Bliss","15-17":2},{"_id":"Green","18-25":1},{"_id":"Bliss","18-25":2}]}]

db.collection.aggregate([{$unwind:"$items"},{$group:{_id:"$items._id",values:{$push:"$items"},id:{$first:"$_id"}}},{$addFields:{values:{$reduce:{input:"$values",initialValue:{},in:{"$mergeObjects":["$$this","$$value"]}}}}},{$group:{_id:"$id",items:{$push:"$values"}}}])