At that age, the most common issue is that one of the pawls has a gummed-up pivot. You can try the blind method of firing WD40 into the shifter repeatedly, or remove the cover, locate the gummed-up pawl, and target it with your WD40 while manually pivoting it with a toothpick or whatever, until it snaps back quickly when released. After that, I suggest oiling it with TriFlow or another light non-vegetable-based oil that won't gum up over time.