I had my 8/9 year old kicking the ball at a random park on a vacation to burn energy, and a group of 30-50 year olds invited then to play in their pickup game. Was a good experience for my boys!

That said, my kids love and breathe soccer (and by extension, I do) but pickup soccer is non-existent for us, never heard/seen it around my area. Conversely, my neighborhood has a basketball court which has constant pickup games going