Searched for Nintendo-Wii, Sony-PS3, Microsoft-360 these are the results.
http://www.esrb.org/index-js.jsp
Ratings explained http://www.esrb.org/ratings/ratings_guide.jsp
Sony
E (26) 66.6%
E10+ (1) 2.6%
T (9) 23%
M (3) 7.7%
Nintendo
E (43) 89.6%
E10+ (2) 4.2%
T (3) 6.3%
Microsoft
E (25) 64.1%
e10+ (2) 5.1%
T (4) 10.2%
M (8) 20.5%
MS has the most Mature games of the 3, Sony the most Teen and Nintendo the most rated Everyone.
66.6 is kinda funny for Sony.