Only 4 years old, and he's got it nailed:

This morning, while lying in bed and feeding Tashi, Oscar comes in and gets under the covers with us:
Oscar:"I'm a big boy now and I don't need Mama milk anymore. I can drink cow milk or soy milk."
Me: "Yeah, and Tashi's getting bigger too. She will probably stop sometime this year."
Oscar: "Then your babas will get smaller and smaller and smaller like mine." (show me his chest).

Pause.

Oscar: (kind of wistfully) "Then they will just be for looking at."