My daughter was 17 when she started college last August. What expenses can you write off from college expenses?

Qualified education expenses for American Opportunity Credit include tuition and books and supplies required for attending the college. The expenses must be reduced for any scholarship money used to pay those expenses. 

You do not deduct ("write off") anything.  Instead, you use your qualified educational expenses (QEE), tuition, fees, books and other course materials, including a required computer to calculate a tuition credit.  It only takes $4000 of QEE to get the maximum $2500 credit, The American Opportunity Tax Credit (AOTC or AOC). She must be a degree candidate and enrolled half time or more.  The parent claims the credit on their return, if the student is their dependent.  Room and Board are not QEE*.  There is even a loop hole available, to claim the credit,  if the student is on scholarship. The college will issue a form 1098-T to report your tuition and fees and scholarships.  They may not mail it. Instead, it may be posted to the student's account at the college web site. 

The fact that she is 17 is not relevant for the tuition credit.  The fact that she turned 17 does mean that she is no longer eligible for the Child Tax Credit. Instead you will get the (up to) $500 Other dependent credit.  It has nothing to do with her being a college student.

*If you have a 529 Plan, room and board are qualified expenses for a qualified distribution from the plan.