any good groomer will be certainly able to cope with a big older dog! Most dogs dont' get groomed at home, so they come to us for life, including right up until the month they die of old age...
I personally start to taper my grooming towards their age when they start to show it. I have a 13 year old standard poodle who comes in, and while he still leaps about and is certainly not about to drop dead, he DOES find standing for 2 hour straight to be hard. I've been slowly over the past year, encouraging him to lay down flat on his side like a show dog to be groomed. He's not hugely comfortable laying like that on the table (less secure feeling I think), but has begun to offer it to me a few times while drying him and doing feet etc now!
With an older dog you just have to be aware that standing for a couple of hours is hard, and standing on three legs while their feet are done is harder still. Most of my older dogs sit or lay down, and I let them, until I need to get underneth them or whatever. Ya just work with them, and they're fine.