* Enrollment: The number of students currently enrolled in colleges and universities in the US is in the tens of millions. The exact number fluctuates yearly and depends on the source (e.g., National Center for Education Statistics, individual college reports). You'd find figures around 17-20 million, but this includes undergraduate and graduate students.
* Age Group: The percentage of 18-24 year olds enrolled in college varies, usually hovering around 30-40%. However, this doesn't include older students returning to school or those attending community colleges or vocational schools.
* Definition of "College": This includes a wide range of institutions: four-year universities, two-year community colleges, vocational schools, and online programs. Depending on the definition, the numbers will vary significantly.
* Time Period: The number of students enrolled changes each year.
To get a precise number, you would need to specify the exact parameters (age range, type of institution, time period) and consult a reliable source like the National Center for Education Statistics (NCES). A simple Google search for "college enrollment statistics USA" will provide current data from the NCES.