What causes people to not have 20/20 vision?

Question

For example I had 20/20 vision for 16 years of my life than when I turned 17 it became 20/25. What causes our eyes to weaken? My doctor could not tell me. Because no one in my family wore glasses.

Answer 1

lots of things.  disease, etc.  but most of the time in the absence of disease, refractive error changes in a young person as you describe above are probably due to:

1) axial myopia progression (thats probably not the case for you IMO based on your description)

or

2) accommodative spasm (most likely scenario IMO based on your description)

a bit about accommodative spasm:

"Q: What is accommodative spasm disorder, and is there a treatment for it? — R.A., Oregon

A: Accommodation is when your eyes shift from far to near to make things look clear and in focus. In other words, when you look from far away to up close, you accommodate.

An accommodative spasm is when your focusing muscles basically develop a charley horse, or get a cramp. This happens when your vision system doesn't work smoothly and comfortably, especially when you get tired or stressed. Some people get tired and stressed very quickly, while others can work for quite a while before this happens.

An accommodative spasm can cause tiredness, dizziness, headache, nausea, distance blurriness, near blurriness, difficulty in understanding what you read, and even double vision.

We treat this disorder with glasses, contact lenses, eye exercises or a combination of these things. The goal is to help you function better with fewer symptoms. Sometimes we even prescribe a bifocal-type lens to help you focus without so much strain... even if you are under 40! — cut & pasted from allaboutvision.com:

http://www.allaboutvision.com/askdoc/mis...

Answer 2

Many things can cause that, however, just the daily strain on the eyes such as computer usage, reading and squinting.

However, for the main question, diseases of the eyes, for example ocular albinism and nystagmus can cause it.