Our son is 2.5 years. He has a high interest in cars and going outside of the house. Every day, whether it is in the morning or afternoon, he always ask us to get out of the house. If we have time, we let him play outside and accompany him. This is usually once per a day. But sometimes, we never let him play outside. I am wondering when we can start to let him play outside the house with his older friends without being accompanied by us. Is there any minimum age? Or any other qualification? Our son cannot speak properly yet and is sometimes hard to understand.
The answer to this depends in part on your locality. In the US, some states have no minimum age for it to be legal to allow your minor to be unsupervised, while others do, as old as 12 in the case of my home state (Illinois). Now, that's not necessarily an enforced law, and undoubtedly is meant primarily to apply to fully unsupervised minors (ie, parents leaving for a vacation), but do be aware of the laws in your area.
Otherwise, I wouldn't say there should be a 'minimum' age globally; each child is different. My 2.5 year old would probably be okay in the back yard without me, although he knows how to unlatch the front gate, so I certainly can't let him alone back there.
How old are the children your chlid will be playing with? How responsible are they? How unsupervised are they (are you nearby and checking on them every few minutes)? These are a few of the things that affect whether this is okay. A nine year old and a seven year old might be sufficiently responsible to be allowed with him unsupervised.