Turning on iCloud Photo Stream If you turn on iCloud Photo Stream, does it automatically share all your old photos with everyone? Or just the recent ones?
Photo Stream doesn't share photos with anyone. It simply makes your photos available on multiple devices of yours (signed in with your Apple ID) for up to 30 days or 1000 photos, whichever comes first.

It does not work retroactively. Only photos taken after it's turned on will be uploaded.