It’s a bit more complicated in dogs, because there is a lot of breed variation – big and small, short and tall, all affects how long they live. If that wasn’t enough, the rate of ageing changes through life – bigger breeds mature slower than small breeds, but then they age more quickly! The Kennel Club has a list of breeds and ageing rates, but as a rule of thumb:
A 2-year-old small dog is roughly the same as 12.5 human years, 10.5 for a medium dog, and nine for large dogs. Then each additional year is equivalent to roughly 4 years for small dogs, 6 years for medium dogs, and 8 years for big dogs. Cross breeds live a bit longer than pure-breeds on average. There’s a handy calculator here
So a 2-year-old Great Dane is 9 human years, but a Jack Russell is already 12.5 years.
By the time they’re 5-years-old, the Great Dane is 45 years, but the Jack Russell is just 36!