My mum's Collie/GSD crossbreed lived to 19½, and might well have gone to 21 had he not developed cancer - so I don't think 21 is outrageously old for a dog.
I do think their figure of 147 is rather optomistic though.
The old 'standard' of 7 dog years per human year is known these days to be over simplistic, and to give inflated results when a dog gets beyond about 5 or 6 years old.
Depending on size, a dog is reckoned to age 12-15 years in the first year - then 7-10 years in the 2nd year - then about 5 per year after that (dogs reach adulthood fast, then slow down)
So 21 works out at about 115 - 120
It's also thought that once dogs get beyond about 10, their ageing slows a tad further, to maybe 4 per year - which could possibly bring another 10 or 12 off that figure...
Still - 100-odd is a damn good innings though
I'd settle for that myself, VERY happily.
And if my dog gets much beyond 15, I'll be happy with that too.