It says "your salary will be at the apprenticeship rate of £2.50 per hour payable in weekly instalments one week in arrears." Surely if its law it doesn't matter what my contract says though?

Hi, I wonder if someone could help me with a minimum wage query. I have been working as an apprentice being paid £2.60 per hour since October 2010. I turned 19 in January 2011 and am still currently employed being paid £2.60 per hour. Its now come to my attention that £2.60 is the apprentice rate, for apprentices under 19 or 19 or over and in the first year of their apprenticeship. Therefore I think I have been entitled to minimum wage since October 2011 but I'm not sure if this means that I had to start my apprenticeship when I was 19 as I was 18 for 3 months. If someone could let me know if I have been entitled to minimum wage and if my employer has to back date and pay me what I've earned? Thanks for any help in advance