Who has to compulsorily file the income tax returns

Hi, Who has to compulsorily file i.t return for the accouting year 2014-15.

            If a senior citizen is having income below taxable limit, including all other sources (including pension), should he need to file the return.

                          If  he receives a penalty notice of non filing, from the I T dept, could he then file the return, and avoid the penalty