Addie began fetch about a month after I got her, so at 10 months. She first started wanting to play when I would be taking a bath.
I guess she just got bored and figured she might as well have me do something productive.
She plays with mini tennis balls for a little while. She gets easily distracted, though, and will see another of her toys on the floor nearby and run toward that one with the ball in her mouth. Then she'll try and take both over to me and never manage as one always slips away, so she'll wrestle over the decision of which one to play with. The ball always loses. She grabs the new toy and runs over to me, wanting me to fight for it and then throw it.