, Craig ... and Best Wishes to your girlfriend on her upcoming Graduation.
I know that under new Immigration Legislation introduced in November 2008, a minimum age of 21 years was deemed necessary for applicants seeking UK Settlement Visas. But I'm NOT sure how the "land lies" [so to speak] as regards Entry Requirements for younger Students... nor indeed whether THEY, in the light of the amended ruling, would NOW be considered as minors.
Consequently, I suggest you 'watch this space'for further guidance from members better-equipped than myself to answer YOUR particular queries.