Is my brother right?

For some background, both of us our US citizens but have lived in foreign countries most of our lives that don’t speak English. Also, this conversation was through text. I think we were mostly messing with each other, but I also feel some seriousness from him.

So my brother (29M) said “bet” to me. I (32F) replied “?!” back to him. He said, “Bet? You don’t know bet?”

Me: 🙄 Him: 😂😂😂 You’re so old. Me: No, you’re trying to fit in with the kids and sound younger Him: No, bet works. And my friend uses it a lot. (He has a new American friend and he is 25). Me: Aren’t you 30? Don’t be calling me old. Him: Language moves, flows, changes. I don’t have to stay in the 2000s.