She was still 11

She was older than Ron.

Yes, but my question is when did she get her hogwarts letter, if when she was 11 she didn't go to Hogwarts

Because in England, if you were born at the end of a year (meaning after sep 1st) you go to school just the year after

She wouldve been born at the start of the school year, so when she turned 11 she got her letter

I thought letters went out in July?

Yes, but at what age did she got it?

According to J.K. Rowling, Hermione was born September 19, 1979, meaning she was almost 12 when she started at Hogwarts. I would assume she got her letter that summer, meaning she was 11 at the time, and was still 11 when she started at Hogwarts, turning 12 about 18 days into her time at Hogwarts.