It's not a requirement. He doesn't have to take one. It's my understanding many kids don't make up their mind till after they've turned 19.
I've pulled together some different things---
Rivals--- Aug 24, 2008 cPanel®
"I'm still kind of open to the idea, but I'm not exactly sure what I'm going to do with that," expressed Heaps. "I could very easily do both, but that's something I need to sit down with parents and talk about, and we'll figure it out from there."
Rivals--- Aug 20, 2008 cPanel®
[And with Heaps likely to take a Mormon mission, albeit a shorter one than the usual two years if he attends Notre Dame, the Irish could put a couple of years between Heaps and Crist as well.]
Salt Lake Tribue--- March 24, 2008 blogs.sltrib.com - The Recruiting Trail by Jay Drew & Lya Wodraska
[Heaps, incredibly well-spoken for a sophomore, said he is "probably" going to go on an LDS Church mission and has told all the schools that have talked to him that he could be leaving after his freshman year.
"That's one of the first questions I get asked: Are you going on a mission?'" he said. "Schools want to know. It is totally understandable."]
So...maybe he takes one...maybe he doesn't...maybe he takes an abbreviated trip shorter than the usual two years.
He also wants to enroll in January. Wherever he goes, I would say, that it's still possible he could get on campus and then decide the time's not right to leave.