Agility

[Stoney666]

What age can one start this?

To complete 18 months. However most clubs have foundation lessons so you can start training (without the equipment and no jumps) much earlier than that.
Contact a local club - it is addictive!

As above, 18 months for competition but you can start earlier than that. I personally don't like to do any kind of weight-bearing, twisting, jumping type exercise to any great degree before 12 months (growth plates still active, more so in some breeds than others) but there are 'puppy agility' type classes out there that will teach recalls over poles on the ground, basic tunnels, walking along a flat dogwalk etc. so you've got a head start when the dog has finished growing.