Hi. I'm online and happy to answer your question today.
The most common cause of your condition is a change in your cornea called arcus senilis
At your age it is usually not an indication of any sort of problem however, when it occurs in younger people (30's etc) it might be an indication that their blood cholesterol levels are too high.
Here is an excellent article on this topic:
and here are a bunch of pictures of the many ways that this condition can look in various people:
What is the take home message? There is nothing you did to cause this, there is nothing that you can take, eat or do to change this and it is nothing to worry about.
Does this make sense to you?
And, now, the obligatory word from our sponsors: :o)
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I guess that you must have stepped away from your computer. I'll switch over to the Q&A system. This system works a lot like 'text messaging' but an email is sent to each of us anytime something is posted to this thread. We can continue to work on your question there..... :)