Yeah vs Yes

aeh72

DS is staring to say more and more words and his latest is "yeah." Not "yes," but "yeah."  He's clearly picked it up from me and DH because when I think about it, around the house, we say "yeah" way more than we say "yes."  I have to admit, it's pretty cute the way he says it so I'm not trying to completely break him of it, but I do feel kind of guilty when he says it, like we've taught him something improper.  I am trying to say "yes" more around him so he learns that too because I do want him to learn how to speak politely.  How big a deal would "yeah" vs "yes" be to you, in particular at this age? And, at what age can you start to explain things like "good manners" and why it is more polite to say "yes" vs "yeah" and they understand?

TIA!