You are correct but that was the most confusing, convoluted, and messy way to get from A to B.I don't think comprehension is your strong suit.
*If you're 10 years old right now in 5th grade and your birthday is today you turn 11.
*Sept you would start 6th grade at 11. Then April 2021 you turn 12.
*You will then start 7th grade at 12.
*If you're birthday is September 27 2009 when you started 5th grade in Sept of 2019 you would be 9 years old and would turn 10 a few weeks later.
*You will then be 10 when you start 6th grade. You would then be 11 when you start 7th grade.
Now I know why you like Trump, the great communicator. Sorry that was mean. May be true but mean.
Sorry. Just a really exhaustive way to explain a simple concept, without giving an actual birth date and mixing a real child with a theoretical child in the same post.