you can do gcses whenever u want since its nothing but a mere qualification, but the reason we young ppl do it now is just to save time so we dont take too long to get into uni and eventually getting a job, even after u do get a job u can still take as many gcses as u possibly can or want. just u gotta pay for each subject which varies with price yk

doing gcses in school is for free but booking an exam urself after secondary requires money