This 11-month old baby isn't even their youngest customer! The ad copy also says:
By the way, Mom, when it comes to toddlers- if they liked to be coaxed to drink their milk, try this: add 7-Up to the milk in equal parts, pouring the 7-Up gently into the milk. It's a wholesome combination- and it works!
Click the picture at Kitchen Retro to see the full-size version of this and other vintage ads. Link -via J-Walk Blog