The Anaheim Ducks' Teemu Selanne had a great season as a forty-year-old. He followed that up with six goals in the first round of the playoffs, which lead the league.
Unfortunately, his season is over. Despite his performance, the Ducks lost their first-round series to the Nashville Predators in six games.
This made me think of other players in NHL history who made significant contributions in the playoffs at an age when their contemporaries are often retired. In no particular order this is a list of some of the best playoff performances by players who have reached their 40th birthday.