Yeah, languages always change over time, otherwise you'd still be talking like it was the Heian period. Most slang doesn't stick for that long though, because kids always want to have their own special words. But some of it stays in the language.
There's still a lot of Japanese I don't understand or can't catch, especially the Japanese a lot of old men speak. It just sounds like mumbling to me.
Thank you for your comments. I also have lots of Japanese language in especially dialects. It is very strange to hear ole people born in Tokyo speak standard Japanese because they don't have dialect.