Why are most Horror film audiences 15 rather than an 18?

As we all know, Horror films are rated 15 and above, but the 15 rating is more dominant than the 18 rating. This is because 15-year-olds are at an age where they are curious about different activities in the world to involve themselves in. However these things are limited, so the main activity that is available and affordable for this age group is going to the cinema. Adults go to the cinema less often than teenagers as they have more exciting activities to choose from when planning an outing, and can cover the expenses.