Basically there is no age limit, any body can start, if they have enough knowledge, but 18 would be a good age to start learning about Forex and after a year or so, if you have learnt, enough, you could start with the Real Account.
I don't think so at such young age one should go for forex trading. High risk is involved while trading in currency pairs. Even at young age if you are going for it then make sure you gain sufficient knowledge about market and get familiar with different terminologies used here.
Although I am not in favor of trading when you are under 18 but you can do it in the student life if your study not disturb with Forex trading. You can make profit with Forex trading which can help you to bear your expenses of study.