In the anime, when Tracey caught his Scyther, they said that he was pretty old - and he did look old. I think they gave him some beard like features, and even though he was pretty strong, I think he ran out of energy pretty fast in a battle iirc.
The surfing Pikachu was also old because plot-wise, the Big Wave occurs every 20 years, and the surfer met the Pikachu when he failed to conquer the last Big Wave. 20 years later in the present, they still surf together. So we can assume that Pikachu can live more than 20 years.