One of my daughters is now in secondary school and is the youngest in her year, she was born a week before August bank holiday. She is excelling in her school work and is in the top groups in a lot of her subjects. It's done her no harm at all to be in with kids almost a year older than her.
However, I will notice the difference in the next couple of years when my youngest boys start school because there's only 11 months between them anyway but they have the added factor of the older one being a September baby and the younger one being an August baby so they will both be in the same academic year. It seems bonkers now as our 3 year old is getting ready to go to nursery and the 2 year old is just starting to speak coherently! at that age there is a big difference in development and we are considering deferring him.