Have the stations changed their targets or have the listeners changed in their tastes, or both? In San Francisco, you have two Top 40 stations: KMVQ which usually does better 25-54 and KYLD which usually does better 18-34. If you look at the largest markets, the leading Top 40 station usually does great 25-54, sometimes #1: Z100 NYC, KIIS LA, KHKS Dallas. In many markets, its an Urban or Rhythmic station that wins the 18-34 prize, while the long-time Top 40 with a popular morning show does better 18-49 or 25-54.
Hot AC stations aim SLIGHTLY older than Top 40, but not by much. It seems these days adult women don't want to listen to music that's all that much different than their daughter's tastes. So a Hot AC station will play most of what's on the Top 40, minus a few rap-oriented or teen-oriented titles. The main difference is Hot AC will go back to 2000 for their gold material, while a Top 40 rarely plays anything more than 2-3 years old. And the younger-aiming Top 40s rarely play anything more than a year old.
So depending on the other formatics of a station, morning show, other DJs, promotions, more rhythmic music, etc. you can aim your Top 40 station younger or older, and more Hispanic or whiter, for lack of a better description. Clearly in NYC, WBMP, the CBS Top 40, aims more Hispanic than iHeart's Z100.