Watching The Fashion Badge, Duggee is described as ahead of the times in his homemade fashion and is clearly shown wearing 1950s/1960s gear as part of a decade on decade montage that also covers the 1980s. Given the lifespan of most dogs I’m splitting the difference been seventy years old and functionally immortal.
From “The History Badge”, we know Duggee has a time machine. He could just be trying his hand at fassion of different eras.