Comment Re:AAVE is a fairly recent development (Score 3, Interesting) 487
To answer your question about age grading, you have to look at a population at more than one point in time. In cases where this has been done (e.g. speakers of central American Spanish), what we find is that young adults have the highest percentage of the incoming feature (higher than both children and older adults). As those same young adults get older, their use of the incoming feature does decline some, but not down to the levels of the previous generation. The 40-year-olds today have a higher percentage of the incoming variant than the 40-year-olds twenty years ago.
Variants in speech can serve as social markers which you use to identify yourself as a member of a group. As a guess, I imagine that the slight decline in use of the incoming variant as you get older has less to do with "learning standard English better", and more to do with it not being quite as important to sound cool as you get older. As a 40-year-old, you probably still wear clothes which identify you as a member of a certain group, but you probably don't dress in quite as trendy a way as you did when you were 20.