Jack would stand up instead of sitting by that age too. He crawled at 5.5 months and walked at 9.5 months. Early, but he was crawling for ages before walking so it wasn't a problem in that way (although 9.5 months is too early to understand simple commands so it is challenging for mum and dad!!).

I don't believe early standing is a problem at all, and they wouldn't do it if their bodies weren't ready I'm sure.