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

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.

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.

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.