I cycled daily to work and/or school for 15 years without a mirror and felt safe enough. Then I tried a helmet mounted mirror when i was 29 (1988) and have not been without it since.

I find it more a matter of convienence in a city with streets full of potholes and other debris. But it is not a safety cure-all as mirrors do have their limitations.

I would feel less comfortable without a mirror while cycling.