"Boomer" is an insult that directly references a protected characteristic, as such it's not only insensitive to use in the workplace, it's illegal. You'd be well within your rights to report this to HR, though I wouldn't recommend that in most cases. At the very least it's worth bringing this up to your manager if it happens repeatedly, phrase it as wanting to protect the company.

Maybe note in writing somewhere that this happened in case you get laid off and need some negotiating leverage to get a better severance. Email to yourself can work well, then it's discoverable.