Should I accept an invitation to give a talk on dated work? I'm a recent math PhD, working in Subject X.  I had the good luck to give a counterexample to a conjecture in Subject Y, using ideas from X.  I don't know much about Y -- in fact I learned the conjecture in a "Y for dummies"-type paper and immediately saw the example.  The people I met in Y seemed to be pretty happy about this.

Now I've been invited to give a talk at a big conference about Y.  I would of course be delighted to do so, but by the time the conference happens my paper will be about 18 months post-arxiv.  I don't really have plans or competence to do further work in Y, and I'm concerned that my paper will be getting stale by the time the conference rolls around.  On the other hand, I could give a basic introduction to X and my example, which I think would be interesting to the audience.

Any suggestions as to how to proceed?  Should I accept and give a dated talk?
I'm a physics PhD student, so our culture is a little different, but I've seen people give talks that were primarily about work published multiple years ago. So if math culture is anything like physics culture (and what I've heard suggests that the time scales are even longer for you), I don't think it's a bad idea. Especially, considering the fact that you have been invited to give this talk, it seems they want you there despite knowing that your work will have been out for a long time by the time the conference happens.

If you want to inject something new into it, you could consult with some colleagues in subject Y and ask them what the relevance of your counterexample is. Then you could finish your talk with an overview of what they tell you.