If you want to annoy someone in their late 30s, joke that all they care about are avocado toast points or beard grooming or running off to Coachella when a project needs to be finished. Because they are millennials.
You can also stereotype Baby Boomers or Generation X(1) and get a rise out of them. Everyone seems to know that these "generation" classifications were entirely manufactured by advertisers, but they caught on and have become part of the lexicon. Advertisers created these sweeping generalizations based on demographics.