What year is a person supposed to write the GCSES?

The typical year for students to take GCSE exams in the UK education system is Year 11, when students are 15-16 years old. Specifically:

In England, Wales, and Northern Ireland, GCSE exams are usually taken at the end of Year 11 when students are aged 15-16.

In Scotland, students take National 5 exams at the end of S5 (fifth year of secondary school) when they are also aged 15-16. These are similar standard and purpose to GCSEs.

Some students take GCSE exams earlier or later than this typical age range for various reasons. Students can take GCSEs in any exam series, there are no age restrictions.

After taking GCSEs in Year 11, students usually continue into Sixth Form/College (Year 12 and 13, ages 16-18) for A-Levels or other further education before potentially going on to university.

So in summary, Year 11 (age 15-16) at the end of secondary school is the most common year for taking GCSE exams across the UK. The qualification level and purpose is similar, just under different exam names between England/Wales/Northern Ireland (GCSEs) and Scotland (National 5s).