Asking if you're "too old/slow for IDPA" is like asking if you're too old/slow for sex. If someone says, "yes, you're too old," are you going to quit?
Sure, stage times might be a little longer, and techniques that work for younger guys might not work for you, but hey . . . get in the game and fight the good fight.
It's like the old bull and the young bull on a hillside.
Young bull says, "Hey, there's a bunch of cattle over there. Let's run down the hill, jump the creek, push over the fence and get us one!"
Old bull says, "Junior, let's walk down the hill, wade the creek, climb through the fence, and get 'em all."
There's guys that shoot action pistol sports at a pace that you could time with a calendar, but you can generally cover both hits on their targets with one paster. That's fine for them, so that's what they do.
The fact that you're out there shooting and willing to work at it makes you an example worth following.