Final answer:
The threshold for vision referral for Grade 1 (age 6) through adult is 20/40 vision.
Step-by-step explanation:
The threshold for vision referral for Grade 1 (age 6) through adult is 20/40 vision. This means that a person with 20/40 vision can see at 20 feet what a person with normal vision can see at 40 feet. It is considered a referral threshold because it indicates a potential issue with visual acuity.