WASHINGTON — Most babies should start eating peanut-containing foods well before their first birthday, say guidelines released Thursday that aim to protect high-risk tots and other youngsters, too, from developing the dangerous food allergy. The new guidelines from the National Institutes of Health mark a shift in dietary advice, based on landmark research that found early exposure dramatically lowers a baby’s chances of becoming allergic. The recommendations spell out exactly how to introduce infants to peanut-based foods and when — for some, as early as 4 to 6 months of age — depending on whether they’re at high, moderate or low risk of developing one of the most troublesome food allergies. “We’re on the cusp of hopefully being able to prevent a large number of cases of peanut allergy,” said Dr.