I am a teenager and I still have perfect 20/20 vision. Now, I'm not expecting this to last, so I have a question.
How do you know when you need glasses? Do you just wake up one day, and your vision is blurry?
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When you can't read anything on the board that other people can, you know you need glasses. I've worn glasses all my life and my vision is getting noticeably worse. It's just a matter of comparing your vision to other people and noticing your lack of ability to see things that you know should be easily visible.