GO FOR IT ......it is a blast!!.....I started my yorkie Millee w some practice sessions when she was one but since she had the LP the vet suggested we do not continue.........so we did Pet Therapy for years ....but when she was 5yrs a local instructor bought the indoor matting that was gentle on the joints so we did it again for awhile....(yorkies only have to do little jumps so it's really easy on them anyway)...last month I completed the beginner class w my biewer Truffles who is 6 mos and he loved it!!!!! It socializes them and bonds the two of you.
Ask your local instructors what you can be doing w your young pup at home before you even start a class........the biggest thing is getting them used to the equipment and even putting a board on the floor and letting him walk across it at home (while praising him w treats )is HUGE...I bought a kitty tunnel also just to get him used to the tunnel.......THE KEY IS GRADUALLY GET THEM USED TO IT SO THE ARE NOT AFRAID OF THE EQUIPMENT.....but get the specifics because you don't want to train him incorrectly and then have to retrain the correct way....
here is a good page that has lots of information including the associations and a super magazine..........
http://www.countrysideagility.com/resources.htm