Currently about 50% of people age 18-21 are in college.
I got data from the CPS for School Enrollment of the population as of Oct. 2011. The data covers both undergrad and graduate.
Here it is broken down by age group :
You can't see it in the chart but surprisingly 0.2% of the 14-15 year olds are in college. I suspect theres more genius kids out there than I realized or its just kids enrolled in a joint community college deal through their high school.
Theres a notable difference in the college enrollment rates between men and women. Here is how it looks side by side:
About 45% of 18-21 year old men are in college and around 55% of women.