How old is too early??

Yesterday we were at Wal-mart getting groceries. David and Ashton were with me while Zander was visiting with Grandma. David severely bruised his heel bone last week playing kickball so he decided to ride one of those motorized carts (yes, that's right, he actually got in one of those things! But I was glad because his limping was really slowing us down).
While I was checking out the groceries David was sitting with Ashton in the cart near the exit waiting for me when (allegedly, because this is second hand) a very pretty teenage girl walked by the two of them. As David sat there quietly he heard Ashton let out a very audible "oooh, whoo." Alarmed, David said "What did you do that for?"
Immediately shy, Ashton denied it, but within minutes admitted he had done it because the girl was "really pretty". What?? Ashton is 4 years old!
When I asked him about it later, I inquired as to why he had a sudden change of heart, I mean I can barely get him to acknowledge the existence of the girls in his preschool class. To this, he replied "I only like grown-up girls."
Great....this means trouble.