Misleading Information about account

I signed up for a contract 2 years ago as young adult. When I signed up, I was informed that their would a fixed rate of R399 per month until my contract expires. The next year, it raised slightly to R421. I thought it was just inflation and didn't think much of it . However, in the last 6 months of the contract, the fee suddenly escalated to R550 per month. I called them to inquire why the sudden increase has occured. Apparently young adults have different brackets, and since I turned 22 they placed me in a different bracket. They omitted this information when I first signed on- All I was told that I'd be paying R399 for 24 months. I tried to cancel or review my contract but they refused. Essentially, they're free to charge extra money when you get older, and they do not inform of you this- and rather just take more money from your account.