My DS6 is big for his age. He is also the youngest in several activities at which he has been in for a few years. He hangs out with kids older than him routinely. I never saw a problem with that - because my DS is so social, I love to see him interacting with kids of all ages and see that the other kids like and include him in everything. In one enrichment activity, there is a 13 year old boy who has been DS's friend for 3 years now and keeps an eye out for him if he needs help.
The interesting thing is that recently, DS has started taking other 4-5 year olds who are new to his clubs/groups under his wing and helping them out.
So, to the OP, give it a few years and maybe your DD will have a younger friend who she is helping out at school too.
