Thank you MITB!! That article answered all my questions. Like this:
"Because the intake of naturally occurring nitrates from foods such as green beans, carrots, squash, spinach, and beets can be as high as or higher than that from well water, these foods should be avoided before 3 months of age, although there is no nutritional indication to add complementary foods to the diet of the healthy term infant before 6 months of age" http://pediatrics.aappublications.or...full/116/3/784
"Around the age of three months, an increase in the amount of hydrochloric acid in a baby's stomach kills most of the bacteria that convert nitrate to nitrite. By the time a baby is six months old, its digestive system is fully developed, and none of the nitrate-converting bacteria remain. In older children and adults, nitrate is absorbed and excreted, and Methemoglobinemia is no longer a concern." http://ohioline.osu.edu/b744/b744_2.html
If you prefer to make your own homemade baby vegetables, an alternative is to choose organic produce. Organics do not use commercial nitrate fertilizers and thus the risk of nitrate contamination/concentration is minimized, but not eliminated. The AAP suggests a very cautious and conservative waiting period, 8 months or older, to make homemade leafy vegetables that may contain nitrates. While this sounds frightening, the fact is that nitrate poisoning comes from contaminated drinking/ground water before it ever comes from vegetables.