They should have started making such decisions long before they turned 17! If they have been properly taught from very early childhood about appropriate attire - school clothes, play clothes, "Sunday church" attire, party attire, etc. - they should start making some decisions by the time they are 6-7 and able to dress themselves fully.
E.g. a friend with a limited budget would take her little daughter, starting by age TWO!, pick out 3 items she could afford, and have her daughter pick her one favorite to purchase.
Long before age 17, and especially if they are paying for at least some of their own clothes!, they should be thoroughly familiar with proper attire for various occasions, how to determine QUALITY clothing design & construction, proper fit, accessorizing, budgeting for clothing purchases, what's a fad & what's a good long-standing & durable choice, and what colors & styles look best on them. They should be pretty good at this by age 12-13, in fact.