Misha didn't start her agility until she was 14 months, however I would like Roly to do the agility class as well, but since he is only 11 months and a much bigger dog I'm probably not going to do too much until he's at least a two years old, maybe just doing ad-hoc class so that he learns what to do.
The members of the club that I go to, regularly compete and very often talk of the wear & tear that is done to dogs joints due to regular training and competition. I had asked about Roly just doing the odd class until he is a year old (afterall its only a month), and I was told that it just wasn't worth the risk. Whilst he might be fully grown, he's muscles and joints are still developing and any injury now however slight would affect his future.
Misha is a lot smaller and a much more gymnastic type of dog, very quick and has less weight to carry to stress her joints (she's only 8kg), but even so her training is limited to the one class a week and we only practice weaves at home at the moment. I'll start doing more practice with regards to jumps when she's 2 years old.