What defines a senior? There is no one answer but you can check whether you might qualify under different circumstances.
To get an age pension you need to be of Age Pension Age which for social security purposes is 65 years for men. Women qualify at different ages depending on their date of birth in accordance with the table below so by 2014 the minimum qualifying age for women will also be 65 years. Beginning in 2017, the Age Pension age will be increased to 67 years at a rate of six months every two years as indicated in the table below. Our Centrelink page has a vast array of information on the subject of Pensions