As someone commented some of the skylines ARE old and it takes time for new construction to substantially alter the "inertia" of an older city's skyline.
It might be interesting to have everyone compute that average age of the 25 tallest buildings for their city.
For Detroit, the top 25 buildings range from 103 to 220 meters and were built over a period of 1924-2009.
Detroit: Average age of tallest 25 buildings: 51 years.
This includes 7 skyscrapers (159m+) and 18 high-rises (103m+)
You can get completion dates from SSP city diagrams or
www.emporis.com
I used the following formula:
Avg. age = ( <current year> X <# buildings> - sum [years of completion for each building] ) / <# buildings>
current year = 2009
# buildings = 25