No where on the WSYL website does it mention 2021, 13, or 7th grade. The rules are to be born after 5/1//02.

If you have paid attention to your own son and his friends, I'm sure you've noticed difference in size at this age group. My own son was 5'7 at 13. He has friends who are older than him that were a full head shorter than him. Welcome to puberty. Some of these smaller boys will be small until 10th, 11th grade.