http://www.fpnotebook.com/PED68.htm
Recommendation on this website is over 4 months before introducing juice.
Birth to 6 months
Avoid honey until over 1 year of age
Risk of neonatal Botulism
Avoid water supplementation
Risk of Hyponatremia
Avoid semi-solid food until age 4-6 months
Infants do not accept semi-solid food
Tongues protrude when spoon in mouth
Unable to transfer food to back of mouth
Risk of Choking
Age 4 to 6 months
Iron-fortified infant cereal (4-8 Tablespoons per day)
Source of Iron and B Vitamins
Begin with rice
Avoid other grains until older than 6 months
Barley
Oats
Wheat
Unsweetened fruit juices (2-4 ounces per day)
Source of Vitamin C
Plain Vitamin C fortified juice
Dilute juice with water 1:1
Avoid other juices until older than 6 months
Orange
Pineapple
Grapefruit
Tomato
Infant able to transfer semisolid food to swallow
Older than 6 months
See Solid Food Introduction in Infants
Fluoride Supplementation if indicated
Water intake may begin
Confirm that fluoride concentration not too high
Initiate dental care
Wipe teeth and baby's gums with soft damp cloth
Interesting the response from a juice company:
http://www.juicyjuice.com/expert/ind_questions.aspx?qa=B81B707C-9C5B-47A4-85BC-3D03CA718F60
What Age do You Recommend Introducing Juice to My Child?
What is the right age to introduce juice to an infant? And what amount of juice would you recommend starting out with? Also, in a 24-hour period, how much is too much juice?
You should not introduce juice to infants before six months of age, according to the American Academy of Pediatrics. It is recommended that you introduce 100% juice after the introduction of solids.