My baby's pedia advised me to test if my baby has any allergic reactions to food as early as he started eating solid food. For citrus fruits, I was advised to give my baby a fruit and wait for 3 days if there's any allergic reaction.
Typically they say around a year old they can get tested, but I know it was an uphill battle to get your child tested.But, It's best to wait till after the age of 2 for accurate testing.