Yeah, maybe it was "only a joke", and - in the more complete context now offered - not intended as condescension. Granted.
But still, in this case, with that customer, I question how responsible it was.
We're dealing with a serious issue here - SD - and the customer in question appears not to have exactly been a SWAT agent with exceptional capability. Is joking around this way really consistent with responsibility, of doing the responsible thing as a gun shop salesperson there to help a customer with SD needs?
Sure, joking around with regulars in the store that you know is one thing, but confounding a person that is not known so well of potentially limited skills seems not to be as responsible.
And even if the salesperson didn't mean the joke in a demeaning way, what's the point of the OP pointing out explicitly that she was the only one that didn't get the joke? That doesn't contribute anything useful, and appears to be condescending. (Yes, we are a public forum.)
Tell the joke and leave it at that. Don't bother us with post-game analysis about who "got" the joke and who didn't that could be construed to convey condescension towards elders and women. Was it supposed to make it funnier that the customer was an older woman? Would it have been as funny if the customer was a 42 year old male farmer or a cop who didn't like the joke?