Leaking AA & AAA Batteries

I've always bought Duracell batteries. I've never experienced any batteries leaking in the package. How old are they? Even with the batteries being in a flash light, the only time they have leaked is when they were over 3-4 years old. When I put batteries in my flashlights or smoke alarms I always write the date on them. The one's in my flashlights get changed out about every 18 months or so, the smoke alarms when the time changes. I don't buy batteries for inventory.