My daughter was a Sept 4th birthday and is now in 7th grade and is in Honors Algebra I. She is also the 3rd tallest in her class. I would have severely regretted holding her back. Size and youngest doesn't matter unless they are NOT ready.
Look at it this way OP. If you think she is ready, start her. You can repeat a grade much easier than skipping. And kids that are bored are usually the ones active and not paying attention... because they don't have to.
My only concern with starting her was that she would always be around older kids in middle/high school and honestly, it meant one less year with her home with me
But she was ready and did awesome!