Canadian Taxes - 18yo to pay cpp?

Hi, I'm a 19 year old student in my 2nd year of University, and I'm just looking over my pay stubs with my mom before we do my taxes. The question I have is this: I was 18 for my entire work period where I was paying cpp on every pay check, should I have been paying this at 18? Can I claim that back on my taxes? or is 17 that I'm thinking of. Thanks for any help!