Short sightedness, or myopia, is a very common condition that causes distant objects to appear blurred, while close objects can be seen clearly.
This happens when either the eyeball has grown too long, or the cornea at the front of the eye is too steeply curved. This causes light from distant objects to be focussed in front of your retina (the light sensitive layer at the back of the eye) and therefore the image is blurred.
Short sightedness most commonly develops around puberty and continues to get worse whilst the eyes are growing, but it is possible for it to develop in young children.
It is not entirely clear why some eyes become short sighted. It often runs in families and could be linked to spending long periods focussing on near objects such as books and computers in childhood as some studies have shown that children who spend more time outdoors are less likely to become short sighted.
Short sightedness can be very efficently and accurately corrected by wearing glasses or contact lenses. Wearing corrective lenses does not encourage short sightedness to get worse, so you can wear them whenever you feel they help.