Vitamin C and other anti oxidants may be effective in delaying cataract formation. Long term clinical studies with thousands of subjects have found some differences in cataract rate between those who took anti oxidants supplements and those who did not, however the studies look at dietary intake, not topical drops. The NAC drops remain unconfirmed, the only studies to date were with very small groups and done by researchers with financial interest in the company selling the drops.
A diet high in fruits and veggies, low in animal meat, low in animal fat and including cold water fatty fish may be protective against cataract, but supplements and drops are likely not. Not smoking and wearing UV filtering sunglasses are two things that will help prevent cataract.
No drop is likely to reverse existing cataract, studies have found slowing of formation but not reversal.
http://www.ncbi.nlm.nih.gov/pubmed/18090901