I don't have a dane with arthritis (yet) but my husky mix Keiko has it. She's almost 10 and started being stiff legged and limping around 6.5 yrs old, too young to start in my mind.
Luckily for her, it's not too horribly bad yet and all she gets is 1000 mg of glucosamine/chondroiton a day. (2 500mg tablets one morning and one evening).
I have heard good things about metacam though. It's a fairly new drug and the last vet clinic I worked at had just started using it shortly before I left (2.5 yrs ago).
Large breed dogs tend to get it younger b/c of their size and the amount of weight, as well as wear and tear on their joints. Gunther is only 3 now, but I'm sure w/i a few years he'll start showing signs as well, which is why I try to keep as much stress as I can off his joints (he doesn't sleep or lay down on hard floors, no jumping or leaping off the ground, eating off a raised feeder, etc)
I hope the metacam is working for you.