The chances of this person (in their very early twenties) having cataracts is about the same as him/her getting struck by lighting....especially if there was no trauma to the eye.
Spots, floaters, bacteria, parasites possibly but cataract? Hard to believe. Especially if it looks similar to these pictures and moves around.
I would get a second opinion. And if you go to another ophthalmologist, be prepared to be diagnosed with a cataract again.
Go to someone who who doesn't specialize to get another opinion to avoid the Maslow's Hammer ("If you have a hammer - everything looks like a nail") effect.