and when my 40 year old niece corrects her 10 yr old daughter's English but can't get her own past participles right.

Ain't is not a word that I use, but it's very old English and IN the dictionary. My husband likes to use it for effect from time to time (yes, he's affected too ) . And one of my college educated grad student sons uses that word now too. Hopefully, again, just for effect.