The normal age for a student to start university in the UK is 18 - over here the term 'college' doesn't mean university, but is a place where you take the bit of post-compulsory education between 16-18 that you generally complete if you want to go to university.
In the UK young people can leave school at 16 after completing their GSCE exams. After that, young people can go into further education (FE) where they can take A-Level exams (generally regarded as the 'academic' route) or other more vocational qualifications, either of which can get you into university. Most UK students who go to uni still follow the traditional A-Level route though, although mature students can take what are known as 'access' courses which are similar to A-Levels but are more geared towards people who may not have many formal qualifications.
To take these post-compulsory qualifications, young people either stay at their school or go on to an FE 'college'. After that, at 17-18, you apply for university (also known as higher education, or HE) and hope you get in!!
Hope this helps a bit!