We love it. There are 3 major organizations-- NADD, Dock Dogs, and UAD. Dock Dogs is my least favorite for a few reasons but also the only one I don't have to drive 3 hrs to compete so we do it.
Look up those orgs and see who has a pool near you. My Dock Dogs club has a pool and you could do try its before joining, you just sign up for a non member practice. The biggest thing is getting them to want to jump from the dock, they don't all just take to it. My two shepherds both did it first try no problem but some others take time. My friend's Aussie needed probably 3 practice times jumping off the ramp before he would jump off the dock but he is a good jumper now.
I've never asked about younger age limits because mine were adult when we started. Last year my old one was still jumping. I don't know about this year.
I actually love it for a low impact way for my big dog to really cut loose. Ever since he was a pup he wanted to jump off tall things and scared the life out of me and I was constantly up his butt not to so this was the first thing we could agree on-- he can run as fast and jump as high and far as he wants and the look on his face in pics is sheer joy