If your birthday is in July/August then this would make you 14/15 in exam season, which is normal. Your age at the time of your exams should not affect you in any way. You're sitting your GCSEs in 2019, so I'm assuming this makes you Year 10 now? If that's the case, your birthday ranges from September 2002-August 2003. If you were born in the summer months like I said, this would not affect you in any way. I'm one of the oldest in my year (currently Year 12) because my birthday is in the Autumn Term, so I sat my GCSEs at age 15/16. This did not put me at an advantage to those aged 14/15.