Mongodb (using mongoose) expiresAfterSeconds (TTL) doesn't seem to work

I'm using mongoose for this.

My Schema is like this:

var UsuarioSchema = new Schema({
    email : { type : Email, index : { unique : true}, required : true },
    //some other fields, but not required, hopefully, for this sample code
    test_expira : { type : Date, default : Date.now, index : { expires : 120 }}
});

When running on the mongodb shell, this is the information that I believe matters:

> db.usuarios.getIndexes()
[
    {
        "v" : 1,
        "key" : {
            "_id" : 1
        },
        "ns" : "dbnamehere.usuarios",
        "name" : "_id_"
    },
    {
        "v" : 1,
        "key" : {
            "email" : 1
        },
        "unique" : true,
        "ns" : "dbnamehere.usuarios",
        "name" : "email_1",
        "background" : true,
        "safe" : true
    },
    {
        "v" : 1,
        "key" : {
            "test_expira" : 1
        },
        "ns" : "dbnamehere.usuarios",
        "name" : "test_expira_1",
        "expiresAfterSeconds" : 120,
        "background" : true,
        "safe" : true
    }
]
> new Date();
ISODate("2013-06-12T17:41:43.263Z")
> db.usuarios.findOne({email : 'someemail@gmail.com'});
{
    /* some fields go in here! */
    "email" : "someemail@gmail.com",
    /* some more fields go in here */
    "_id" : ObjectId("51b8b087de01a2ec28000002"),
    "test_expira" : ISODate("2013-06-12T17:31:51.156Z"),
    /* some yet more fields go in here */
    "__v" : 0
}

I haven't tested for how long it goes without getting deleted, but even the docs state only a minute overhead should be what one could expect, but not over 15 minutes for a document that should last only 120 seconds.

I'm unsure of how to deal with this. Help will be greatly appreciated :)

EDIT: Version of mongodb being used is v2.4.4 Version of mongoose is 3.0.3