At what point in the progression of cataracts does a human eye become cloudy?

More in elderly. The clouding of lens gradually occurs and is seen more with increasing age usu starting late 50s/ early 60s. Patients with diabetes, trauma to eye, on steroids or with genetic component may develop cataracts earlier in age. There is also a more rare congenital and pediatric cataract.
Varies. Everyone is different. I've done non-congenital cataract surgery in 30 yo's, and 95 yo's. Generally speaking, i think most people between the age of 30 and 40, the lens starts to get cloudy, but its usually not visually significant.
Subjective. Depending on your visual needs, some people find early cataracts a problem (i.e. Painters, engineers), while others don't notice until it drops their visual acuity more.