where you can go depends on where you live.
I dont think its 5-7 is too old to start
and their are no dogs too small to play agility lol I was at an agility fun match last night, their was a chi named kia there who was quite litterally beating out the border collies! there was also a papillon who beat out the BCs as well lol but it was that chi that everyone fell in love with, she was no bigger then my rabbit! lol and shy is NOT a drawback, a lot of times people with shy dogs try agility because its on their own there is no other dogs in the ring with you and the dogs are not supoosed to go say hi to the judge lol if anything I would say its having the social butterfly dogs that are the drawbracks! lol they all want to go see the people on the side lines.