Do You Have To Be 17 To Apply For A Provisional Driving Licence Or Can You Be 16?

You can be 16 and apply for the license.  16 and 10 months, to be exact.  But you cannot use the provisional license -- use it to be a learner-driver on the road -- until the day of your 17th birthday.  

So there's no harm in picking up a form once you're about 16+9 months, and filling it in, getting your photo done, etc.  But don't bother sending off the form until you're 16 and 10 months.  And you may get the provisional license back weeks before you can actually use it.

The notable exceptions to the above are for disabled people, and for mopeds.  If you are registered disabled and receiving the mobility component (higher rate) of Disability Living Allowance, you may be allowed to get a provisional license from your 16th birthday.  I am not sure, but I think you can take the exam to get a full (not provisional) license earlier than 17, too. Best to ask at the test centre, though.

Also, people can get a provisional license for driving mopeds from 16.  But you can't even take a moped on the road until you pass the Compulsory Basic Training test.  Plus (under relatively new rules) you can't get a license to drive the moped until you've got a full driving license.

Those are the rules in the UK.
Elsewhere in the world all sorts of different rules apply.