I think there's no way to really say that someone is or is not safe to drive based on age alone. My father's godmother lived to be over 100 years old and she still drove at 100. She lived in a lodge (meals and housekeeping provided; you may require assistance with getting up in the morning and going to bed in the evening but otherwise you have to be pretty independent) and that was mainly because she was widowed and had been living alone on an isolated farm. Her sons were concerned that in the case of an emergency, especially during winter (living in north central western Canada), it would be hard to get to her quickly. I think she was still very functional and it was just the logistics of her home that were at issue.
On the other hand, there are younger people who should not be driving for a variety of reasons.
One thing to do with someone you think should give up driving is to sit down with them and price out what it costs for road tax, inspections, insurance, fuel, repairs, etc. in a year. It all adds up after a while. The money saved from not having to pay for those things would pay for a LOT of taxis, Ubers, trains, buses, etc. Of course the one caveat is that you would have to be in an area that has access to these services.