Will dyslexia start showing when your 13?

Dyslexia: Not typically. Usual age of DX is 7-8 years of age. A full psycho-educational evaluation can answer this. Public schools administer this for free. As your school psychologist.

Late onset: dyslexia is a disorder of symbol identification. 13 is very late for it to present. You should have an evaluation with a reading s pecialist. It is likely that it is the complexity of the matrial you are now required to read.