This is not precisely specified. Some people with congenital diseases start wearing contact lenses since childhood. Whatever age the person may be in, he/she is considered old enough to use contact lens if he/she can manage the lens on his/her own. For children not old enough to put on or remove the lens by themselves, adequate help should be available from adults.

The most important difference is the permeation of oxygen through the lens. Silicon is added to the R.G.P. lens to enable the oxygen to penetrate. Only R.G.P. lenses that can provide the nutritive elements in the atmosphere to the cornea are manufactured these days to prevent possible ill effects due to hypoxemia.

Normally, it takes the average person about one week to one month to get used to R.G.P. lens. But the period can change depending on the physical condition of the person wearing the lens. One month is usually good enough for anyone to get used to R.G.P. lens. A person wearing R.G.P. lens may first experience one or more of the following. However, the symptoms may differ depending on the physical condition of the person. 1. Feeling of being under an alien substance 2. Being dazzled 3. Excessive tears 4. Lens move around 5. Increase of blink 6. Being dazzled at nighttime 7. Unstable visual acuity 8. Blurred, glossy light

Germs can easily contaminate saline solutions that did not undergo preservative treatments. Once opened, the saline solution should be used as soon as possible. However, the saline solution should only be used as a rinsing agent. The saline solution is not recommended for use as eyewash or for the preservation of contact lenses.

When the lifespan of the lens is over or if the lens had been improperly maintained, you may find some alien substances on the lens. Too much secretion may cause the same problem. The lens may feel dry and hard, or you may experience congestive symptoms. The lens should be replaced with new ones in these cases.

There are two possibilities. One, the lifespan may be over. Two, the lens might have been used in a dry environment or your eyes might be dry. Use artificial tears or wash the lens again before use.

When re-using the contact lens after a long period of time, consult a specialist to see if the lens is still good for use.