Just wanted to add, IME dogs with shorter life expectancies arent seniors at 6. From the dogs I have had, their senior period and aging is just a bit faster. Like with my lab mix, he started slowing down a bit around 9 but was still active....now at 13 I would consider him senior. So 4 years of a gradual decline. Giants, again IME, dont do 4 years of decline...so a 10 year old estimate is not a senior at 6. Does that make sense?
I have known plenty of older danes, 9, 10, and up who still act pretty much the same as always. They are still active, still playful, still very much alive.