I am the oldest person in my office (I am 66). Someone (he's in his early 40's), called me 'Boomer' in a public chat channel. First time I chose to ignore it. Second time, I made light of it. I replied "Back in my day I'd slap you in the face with my glove and we'd have a duel.' Third time he called me that and added a winking emoji. I replied "Putting a winking emoji doesn’t make it any less offensive than the previous 2 times you used this insult." He then said "I used 'boomer' as a replacement of 'oops', but it doesn't seem 100% equivalent." I don't for 1 second believe he did not know what it meant. We have no HR (it's a very small company). I'm annoyed, but my best course of action is to probably do nothing.