As Alexander pointed out, there is a correlation between Age and your chance of being a Top or a Bottom.  The chart above shows the relation between Age, Height and Weight to top/bottom-ness.  The dotted line is “Average,” so if a bar is below the line there are more bottoms in that group, and if the bar is above the line there are more tops.
Younger folk are more likely to be Bottoms
After 36, the chance you’re a Top increases yearly…until you’re 60.
Men under 5’10” or over 6’4” are more likely to be Bottoms.
Guys lighter than 180lbs or heavier than 280lbs are more likely to be Bottoms.
Any ideas why guys over a certain size start trending more bottom-ish?