Here’s a bunch of words that either didn’t exist at all, or didn’t exist in their current form/meaning when you were growing up:

smartphone, app, emoji, meme, livestream, crowdfunding, cryptocurrency, blockchain, NFT, ransomware, selfie, vlog, podcast, cloud computing, Al, algorithmic bias, social distancing, contact tracing, microaggression, cancel culture, virtue signalling, gamification, enshitification, deepfake, influencer, cybersecurity, carbon footprint, microplastic, drone, smart home, loT, cryptocurrency, biohacking, wearable, crowdsourcing, clickbait.

But I bet you could understand someone that used most if not all of those words right? Because you learnt them, even after adulthood? You can learn and understand these new words too.

Also I find it incredibly hard to believe they couldn’t understand you. Even if the young generation uses a ton of slang with each other, they interact with teachers, parents, grandparents, media such as TV and movies etc I could go on. Unless you were intentionally using very old or foreign slang heavily I find it near unthinkable they actually couldn’t understand you.

Edit: I just noticed you’re the same person I replied to in another comment. I wanna be clear I wasn’t seeking you out or something, I barely look at user names, it was coincidence.