You're best deciding on whether you want to study a science or humanities based degree subject. A science access course won't provide you with the necessary knowledge to study History, and vice versa. You should pick the one that most interests you, so have a look at various degree subjects offered at universities you'd like to go to. Perhaps you should look at what your favourite subject was at GCSE level and whether you preferred writing long essays for English or were better at Science, that should show were your strengths lie academically. Then once you've decided, but may not be sure on the degree subject, pick an access course that is very generalised, so 'Access to Science' or 'Access to Humanities and Social studies' rather than a particular one like 'Access to Law' or 'Access to Medical Science', for example. Or if you know that you'd like to study Law or business, pick an access course specifically for that subject (if they offer it).
Probably not, no. Most colleges make their access courses 19+ or 21+ to keep them for mature students and for funding reasons. At 18 they'd probably push towards A-levels or BTECs which would be mostly younger people in class.