I don't like to start serious agility training until my dogs are 18 months or so, so I'd tend to start trialing around 2. I think that's about when I started with Tess, though I can't remember for sure. Pirate, I know I started his training a little younger than I wanted, because I had a good foundation class available to start in. So he was 18 months when he started trialing. Not my ideal. He still hasn't started obedience. And I can't remember when I started obedience with my other dogs.
I don't see any reason why you shouldn't start dock diving with a younger dog, though. It doesn't have the repetitive actions, or intensive training. And it's low impact.