The concept that dogs age seven years for every human year is a myth, according to scientists.
Researchers from the University of California say puppies are already middle-aged by the time they reach two years old.
According to the university's research, three-year-old dogs are closer in age to a 50-year-old human - despite many owners regarding a three-year-old dog as young.
The study analysed the ageing process in dogs by focusing on DNA methylation in 104 Labrador retrievers spanning a 16-year age range.
Scientists are able to track a dog's biological age by analysing marks in the DNA that change over time as a cell matures.
The researchers then compared their findings to 300 humans, finding a "nonlinear relationship" between dog and human ageing.
A two-year-old Labrador's DNA is roughly equivalent to that of a human in their early 40s - rather than a 14-year-old, as suggested by the largely accepted formula.
The results also demonstrated that dogs age more slowly over time, meaning the maturity of a 10-year-old Labrador could be compared to that of a 68-year-old human.
The researchers also revealed the ageing process of eight-week-old puppies showed similarities to that of nine-month-old babies.
The study, published on biorxiv.org, has yet to be peer-reviewed.