As long as you are able to safely drive at age 70 and beyond then nothing happens to your drivers license when you reach 70 or any other age.
As long as your drivers licenses is valid and not expired or suspended you can continue to drive at 70 years old and beyond as long as you can safely do so.
I know a 98 year old guy that still drives himself and he's a great driver and I would trust him more than I would trust some new drivers.
It's not just the age that determines when someone should quit driving but whether or not they have a good mind, good reflexes, good eyesight etc.
As long as they are in good enough health to safely drive they can do so without any issue.
But if someone gets to the point where they cannot safely drive anymore then they should stop driving.