About the only dietary sources of DHA are fish oil and human milk. Commercial infant formula manufacturers are studying the issue of adding DHA to formulas, but as far as I know have not yet done so. Adults probably do not need preformed DHA in their diets because they readily synthesize it from linolenic acid which is present in a range of foods.
Soybean oil is one of the richest sources of linolenic acid.
Both are in the n-3 or omega3 class of fatty acids. Babies, or at least premature babies, probably need a source of DHA in their diets. Such an accumulation of DHA in brain tissue suggests an important role for DHA in development. Research with laboratory animal babies has shown that feeding them diets low in linolenic acid or DHA resulted in impaired visual function and poorer learning ability. Monkeys fed diets deficient in DHA exhibited behavioral abnormalities.