How to get the double quotes arround number typed field from MongoDB aggregation query result?

Scenario: I have a collection 'People' with following documents

       {
         "_id" : ObjectId("512bc95fe835e68f199c8686"),
         "name": "David",
         "age" : 78
       },
       { "_id" : ObjectId("512bc962e835e68f199c8687"),
         "name" : "Dave",
         "age" : 35
       }

When I query using following code from Node.js

db.articles.aggregate(
    { $match : { author : "Dave" } }
);

The output will be like:

       { "_id" : ObjectId("512bc962e835e68f199c8687"),
         "name" : "Dave",
         "age" : 35
       }

Issues: The above is just a sample of the actual scenario, I want the 'age' filed value to be embedded in double quotes i.e for above quoted example it should be like "age": "35".

That is full resultant document should be like following:

{ "_id" : ObjectId("512bc962e835e68f199c8687"),
     "name" : "Dave",
     "age" : "35"
   }

Consider I have huge number of documents how efficiently I can achieve the same to get the desired output?

Question: Can someone help out with bright and efficient way to achieve the same?