Both are acceptable:
 a. Throughout the United States a large number of young people enter college
b. Throughout the United States, a large number of young people enter college
The comma in [1b] is called a delimiting coma, which means that it marks the boundary of 'a subclausal constituent that is set apart from the main part of the sentence, usually indicating that it is in some sense less central to the message' (CGEL, p. 1745). The element that the comma sets off, throughout the United States, is grammatically a preposition phrase (PP) whose grammatical function is that of an 'adjunct in clause structure'. The grammatical function is the same regardless of whether the PP is delimited by a comma or not. Such commas are optional, and the choice of whether to use them depends on a number of factors (CGEL, p. 1746):
With adjuncts, there is considerable variation as to when delimiting commas are used: this is the area where the contrast between the heavy and light styles of punctuation is most evident.
The main factors influencing the use of delimiting punctuation are:
 i length and complexity of the constituent
ii whether or not there are punctuation marks nearby
iii the linear position of the constituent
iv the semantic category of an adjunct
v the possibility of misparsing
Other things being equal, a short simple constituent is less likely to be marked off than a long complex one (e.g. one with the form of, or containing, a subordinate clause).
As it happens, published literature has related examples both with and without the delimiting comma. My impression is that while the versions with the comma are significantly more common, nevertheless the versions without one are still pretty common, too.
Without the delimiting comma
Throughout the United States young people are actively opposing the policies of the United States government. (source)
Throughout the United States people had extremely different opinions about prohibition. (source)
Throughout the United States there are millions of people of all ages that participate in a wide variety of sports and recreational activities. (source)
With the delimiting comma
Throughout the United States, there are thousands, indeed millions, of inventors and entrepreneurs developing new ideas and technologies to solve problems and fulfill consumer demands. (source)
Throughout the United States, people protested the war. (source)
Throughout the United States, people who want to become licensed or certified as counselors are required to have at least a master's degree in that field, with specific coursework included in their programs of study. (source)