Using the vast amounts of data available via his online dating company Christian Rudder answered the question for heterosexuals. He told us what we already know but didn't want to admit.
Below is the graph of what aged men, women find attractive. You'll notice somewhere in their late 30s women start finding slightly younger men attractive.
Below the fold is the chart for men, or what women they find attractive.
http://fivethirtyeight.com/...