both my boys were playing out by the time they were 5 years old, which I know is on the young side. Titania, I think you are perfectly within your rights to say to a 5 year old, 'no playing out'. You can invite friends to your house, instead.
Allowing your children to play out is a very, very personal decision - you look at your street, look at the children who play out (do they fight, do they play dangerously etc) and go from there. I know it's quite possible my boys would not play out if we lived elsewhere.
But if you feel your area is safe, the children who play out are well behaved, nice kids on the whole, and your child sensible and well within the age group of the playing out children, I think your child would be justified in feeling you were a bit mean if you did not ever let them out of the front door alone.
IMO if your child will be travelling alone to and from secondary school, it's a good idea that they have some playing out experience before they move up, just so that they are used to being alone in the local area. My 10 year old will be catching buses by himself in 9 months time and walking all over the place as our schools are quite far away. I feel safer knowing he has played out and has some street sense.