KOLA in the Inland Empire has some similarities to KKHH, given that it too plays a lot of 90s (and even beyond) for a Classic Hits station. It is more to go a different direction than KRTH, however. I cannot say if the white median age is lower in this Latino-majority market.The core of a healthy Classic Hits station is 35-54, so I was moreso referring to the average age of the non-Hispanic whites in Houston possibly being lower than other markets, which may be why the market can't support a "pure" Classic Hits station that is heavy on the 1980s (regardless of whether it's pop leaning, rock leaning, or more of a balance) and why KKHH has gone with a playlist that is much more agressive with 1990s music than the typical Classic Hits station. This could also be a factor as to why Atlanta doesn't even have an Adult Hits station, let alone a Classic Hits station.
WLS-FM in Chicago leans towards rock music, but the median age among whites isn't particularly low. WLS-FM's musical direction can be attributed to the rock lean of the WLS of old and possibly the distaste of disco/dance music among some listeners. Also, WJJK in Indianapolis and, to a certain extent, WOMC in Detroit have a rock lean, but Indianapolis and Detroit aren't known to have a particularly youthful population.
As for Atlanta, note that Atlanta lacked a mainstream Top 40 from 1992 to 1999, and Star 94 was successful as a result of its Adult direction. In addition, Q100 feared Star 94's success during the late 2000s and went with an Adult direction, away from its Mainstream approach in 2001-2005. So Atlanta not having a [full-powered] Classic Hits or even an Adult Hits is likely attributed to non-urban listeners, at least those within the 35-54 demographic, not caring for rhythmic-oriented music.