I don't cull based strictly off of a certain age as long as they still raise a good calf, breed back on time, and are in good physical condition. I've got a little 14 year old cow that raised a set of twins last year and bred right back on time, and she still is one of the first ones there when I go to move them to another field. I'll have some start showing their age before they're even 10 years old, so some age a lot better than others. Most of the time mine will get culled before they make it to their teens based off of calf performance or some type of physical issue. All that being said, if I was deciding between two and had to cull one, age would be one thing that would factor into my decision.