At what age do you think signs of a child's future behavior, good or bad, starts manifesting itself?

I visited a friend yesterday who has a 15 month old grandson. I let the kid carry my keys around for a bit.
Then Gramma offered him a sippy cup of water. He squatted down, laid the keys on the floor, stood back up and accepted the sippy cup. I was kind of amazed that he laid the keys on the floor instead of just dropping them. I wondered if this was indicative of anything. That family has a history of turning out well-behaved children.

Conversely, we had an acquaintance over not long ago who has a grandchild about the same age. This kid was going around knocking stuff off of tables just for the hell of it. I quickly gathered up everything he could reach and put it up high. That family has a history of turning out poorly behaved children.

So at what age do you think future behavior starts showing up, and why? What is there that a parent could even do in the first 12 months to cause that to start manifesting itself?