For me, it depends on lots of factors:
- how mature an 11yo she is
- how mature her friends are
- how many of them there will be
- how busy the park is
- how close the park is to the shops
- if the park has known dodgy areas
I have let my DD11 go to our village park with her older sister, or with a friend, since she was 10. It's a very safe park with lots of people she knows nearby that she could go to if she needs help. She isn't neurotypical but she is actually very sensible in a crisis situation - more so than her neurotypical older sister!
When she went into Y7, I started off allowing her to go into town after school for an hour if she went with 2 friends. (This was because her 'best' friend is unreliable.) This gradually increased to 2 hours. Now, she is allowed to go into town with 1 friend or with her sister after school and there is no time limit as long as she has pre-arranged where and when I pick her up. She can also meet a friend in town at the weekend. However, I don't let her go to the park in town because it is a little distance from the city centre and has some dodgy areas.
However, my DD13 is allowed to meet friends in the park because she is older and her friends are very trustworthy and I know they keep away from the dodgy areas.
They both have software on the phones that allows me to 'track' their phones and are not allowed out if they don't have their phones with them, sufficient charge and the location switched on. They know if they switched it off, they'd have going out privileges revoked for a time. They also know if they are late, they will be allowed out for less time next time - they earn the trust to stay out longer/go further over time.
I do tend to have to ferry them about as we live in a village with a very limited bus service so my reasoning to them is that I am doing you a favour by driving you to and from all the activities you want to do, you repay that favour by being on time and keeping to the rules I've set.