Well having done agility with my german shepherds and my aussies, The aussie had more longevity when it came to agility..My aussie is 13 and still going strong.
All but one of my german shepherds have lived until 13 almost 14 years of age, which is not long enough in my opinion, but a good long life for a large breed dog.
It's never to late to take classes as far as I'm concerned, I don't think I'd start competitive agility with a 6 yr old dog unless it was REALLY good at it, was in excellent health joint wise..I would however, have no problem doing obedience with a dog that age.
Danger Danger vom Kleinen Hain aka Masi
"Angel" Jakoda's Bewitchen Sami CD OA OAJ OAC NGC OJC RS-O GS-N JS-O TT HIC CGC
"Angel" Steinwald's Four x Four CGC HIC TT
Harmonyhill's Hy Jynx NA NAJ NAC NJC RS-N JS-N HIC
Jakoda's Jagged Edge