What causes to have cataract???

I just had a cataract surgery a week ago. One eye at a time. Next surgery will be my left eye. My friend told me that I'm still to young to have cataract. I don't consider young myself when I hit 50 years old.
I don't know if staring to long on the computer screen can cause cataract.

Cataracts form for a variety of reasons, including long-term ultraviolet exposure, secondary effects of diseases such as diabetes, or simply due to advanced age; they are usually a result of denaturation of lens proteins.

it is clouding of the eyes' lens - but I don't know exactly why it happens

It often is hereditary, but that is the ones you get in your older years, 70 or 80's. The ones you get young, like you and me, just happen. I had both mine done in my 40's. The doctors said I was just "lucky" that way. 10% of the population gets them in their 40s-50's. It is much more aggressive than the older people one. Theirs often take 5 years or so before it is ready for surgery. This type is usually much quicker.

My husband has had a cataract since in his early thirties. He still has it. The doctor has not suggested removing it yet. I think heredity has more to do with it.

Have a look at this link

Hmm not uncommon at all at this age and it is not "mostly" hereditary, though some cases can be.

UV rays, general degeneration, tendency (this is not heredity)

Staring at the computer screen does not cause cataracts. Thye can be due to several things.
Age is one, the lens just becomes cloudy due to the agibg process.
Diabetes can also cause them. My friend is 46 and has had both eyes done in the last 2 years.
Severe eye infections can damage the lens too (can affect any age).
There are other causes too but I cant remember them right now :)

Nothing you can do can cause a cataract, except if you take steriods like prednisone.Then sometimes these can cause cataracts if taken for a long period of time. Everybody develops cataracts if they live long enough. Usuing your eyes do not cause cataracts.