I can't say I'm good at it, but the best advice I can offer is to be genuinely interested in them. If they are telling you that they love classic rock then you need to reply. That reply doesn't have to be about you in the slightest. Instead, ask them what is about classic rock that they like. Ask them what years and artists they consider to be classic rock. Look for places where you still do or did align with them. You can then segue and say something like, yeah I really do/did love artist 'x', too. I've also started listening to a lot of (insert genre) because I've been looking for more variety than the same 100 classic rock songs I've been listening to for the past 50 years.
Of course, in the above dialog, there is no requirement that you ever share your current opinions about classic rock, if they don't ask. You can always keep asking them questions about them. What artists do they wish they could have seen in their prime? What artists did they use to love/hate, but have changed their view. What do they like about classic rock that they don't get from other genres. As long as you remain genuinely interested and engaged you can go on forever and they'll love you for it.