When you get to 70 you have to renew your license every 3 years. At 70 you have the option of keeping your full entitlement with a medical or being restricted to driving vehicles up to 3500kg without a medical. As my motorhome was under 3500kg when I reached 70 I never bothered with the medical. So for the last 12 years I just signed that I had no problems.
Personally I would like to see everybody have a medical and get an eyesight test every time they renew the over 70 license. I've noticed over the years my observation and anticipation is nowhere as good as it was, I know this but many drivers don't.