[Codein]

I got taught about basic trig, including cos/sin/tan functions, at around 13 years old and was a recurring subject until I'd finished school, which was about 3 months ago or so.

As soon as it comes up once in your syllabus, it's more than likely to reoccur in the future. For example, I'm taking Maths at Sixth-Form (which is like Year 12 and 13 of school) and trigonometry makes up a good part of it.