There are various different types of cataract and they can all progress at different rates, so there's no hard and fast answer other than the 'wait till they impact everyday life'. What that means will also be different for different people. If you don't need to drive, for instance, maybe you can wait longer than someone who is reliant on driving to get around.
Opthalmologist I saw in UK said that generally speaking the longer you can wait (the older you are) the better. Something to do with viscosity of fluid inside the eye and the risk of it tugging on the retina and causing a tear when removing the old lens, I think.
Eye morphology is also a factor: people who are very myopic may have a greater risk of retinal or other damage, although statistically it's still quite rare.
Opthalmologist also said that people who expect return of perfect 20/20 distance vision without glasses after cataract surgery are more likely to be disappointed than people who expect greatly improved clarity of vision, but possibly only getting 20/20 still with glasses.
He also said that while nowadays you can get varifocal implant lenses, he usually recommends simple fixed power implants, especially if the degree of correction required is substantial.