Has the Walking Dead canon addressed zombie ageism and featured old people zombies? Are the zombies that used to be elderly people less robust than the zombies created from younger people, or are they just as functional?
There hasn't been any mention as to whether elderly people make weaker zombies. AFAIR, we haven't seen any elder person turn; or we have seen and there wasn't any mention of it. But we have seen child zombies, so I guess elderly zombies exist too.
But, as with child zombies, we don't know if they are weaker or not since there hasn't been any such mention.
Now, as far as zombie ageism goes, there has been another question here: Does a Walker's senses diminish with time?
Where I answered with a quote of Kirkman's on Reddit:
OddGuyOut: In the beginning of the show we saw walkers do things like using a rock to help bash the doors in or turning a door knob, is there a reason we've stopped seeing them do that?
Robert Kirkman: Older zombies are less together and capable or doing things like that. Fresher zombies, which there were more of in season one, are able to do more than older, more rotted zombies.
Which simply says that, yes, zombies do age and they are not as strong as they used to be.