Signing gym contract when I was 17, now 18 and want to cancel it?

I signed a 12 month gym membership contract last year when I was 17 without them asking any questions about my age, I am now 18 and wish to leave the gym but the contract says I will have to pay £70 in order to cancel it before the 12 months finish, as I was 17 when I signed it, can they still enforce the £70 payment?