I could do that at 13 too. My opthamologist didn't think it was any big deal. (I guess he would have felt differently if I couldn't get my vision back.)
The eye is like an onion, it has layers. When we're younger, it has fewer of these layers, and is quite flexible, and we can often (at will) change focus so things are blurry. As we age, more layers are added, and the eye is less flexible (which is why people often need reading glasses at 40 when they never did before), and slowly but surely you can't focus and unfocus at will any more.
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