The American Optometric Association recommends:

Children:

Birth to 24 months: at 6 months of age

2 to 5 years: at 3 years of age

6 to 18 years: Before first grade and every 2 years thereafter

Adults:

18 to 60 years: every 2 years

61 years and older: annually

If a child has certain risk factors, such as prematurity, oxygen at birth, a difficult or assisted labor with low Apgar scores, or if an adult has diabetes, hypertension, a family history of ocular disease, a history of cancer (especially lung or breast cancer), as well as many other conditions, more frequent eye examinations are recommended. Certain medications may affect the eye or vision, and patients taking these require more frequent eye examinations.

My own office policies and recommendations are:

Children: I feel comfortable examining children's eyes at age 4 or 5. Pediatricians usually screen for the eye diseases that may affect a newborn and can detect eye problems that tend to develop in the early years.

Things to look out for in infants and young children that may be a problem:
1. Cloudy cornea, or cloudiness in the pupil of the eye.
2. Tearing of one or both eyes.
3. Sitting very close to a television or holding books very close to the eyes.
4. Improper alignment of the eyes. (appear crossed) Many children are born with a fold of skin in the inner corneas
of the eyes that gives a false impression of crossed eyes. This fold diminishes as they grow and the eyes will develop a normal appearance.

If eye problems are suspected before age 4, a Pediatric Optometrist or Ophthalmologist should be consulted. They have special training and equipment to properly examine and treat problems in very young children.

I think that children and teenagers should be examined on a yearly basis, as these are the years that a prescription is most likely to change rapidly. It is not uncommon to see rapid increases in myopia from the 4th or 5th grade until the 9th grade. If I see a large change in prescription, I recommend to see the child in 3 to 6 months, depending on how much of a change.

Adults (ages 20 to 60): I follow the AOA recommendation of 2 year intervals. Exceptions are patients who have the above mentioned risk factors. I would also like to see an adult sooner than 2 years if their prescription changed more than normal or in an unusual way. Sometimes a patient may develop one of the normal aging problems (examples - cataracts, early signs of macular degeneration, etc.) prior to age 60 and I would want to see them sooner than 2 years.
The chance of developing certain eye diseases increase (cataracts, glaucoma, macular degeneration) when a person is over 60 years old, so I would like to see them annually.

Contact lenses wearers are much more likely to develop eye problems and need to be examined more frequently. A contact lens wearer should be seen annually, or more frequently if sleeping overnight in lenses, or if they have conditions making them more likely to develop eye problems.