Excellent question.
Yes, for the most part, all pokémon have longer lifespans than their animal “counterparts.” Therefore, yes, a wild rat may live for about one to two years, but a wild rattata may live for up to ten years—sometimes even fifteen if allowed to evolve and if taken care of well.
On that note, evolution can generally extend a pokémon’s life span, but it should also be noted that a pokémon may live twice as long if in captivity, rather than in the wild. For example, whereas a pichu may live for four years in the wild, they live for eight years in captivity. Captive pikachu may live for about fifteen years (same as raticate), and captive raichu may live for over twenty.
Keep in mind that all of these are estimates, of course. There have certainly been records of pikachu outliving some raichu (that is, a pikachu may live for over twenty years), and of course, just because you find a four-year-old pichu in the wild doesn’t mean it’s near death. It all depends on a variety of factors, including and especially the quality of life.
And yes, there are also pokémon who live for so long we’re not entirely sure what their lifespans are. This is especially true for the legendary pokémon, but dragon-types are equally notorious for this. For example, some families in Johto have passed down the same dragonite from parent to child for generations, and that dragonite may not look a day over whatever young age it had reached when it evolved.
The Pokédex According to Bill 