Return difference in date's mysql

See more:

PHP

MySQL

CodeIgniter

I wish to return the difference between

start_date

and

account_add_date

For Example:
The user added an account on 1st December 2015 and started the account on 1st March 2016 so the difference is 3 months.

My query is returning nothing, what is the problem ?

What I have tried:

function get_age() {
$mem_id = $this->session->userdata('id');
$query = $this->db
->select("12*(YEAR('account_add_date') - YEAR('start_date')) + (MONTH('account_add_date') - MONTH('start_date')) AS differenceInMonth")
->where('mem_id', $mem_id)
->get('bank');

return $query;
}