a simpler answer to the conundrum.....would you let your 8 year old child ride off road or on a mainroad without one.
It's not an answer to the question, because a child of 8 and a child of 12 and an adult are very different. There are main roads and main roads, and off road can mean a whole range of things.
There are bridleways and tow-paths I'd be happy to ride helmetless with a well-coordinated kid. An 8 year old probably shouldn't be on anything that could be called a main road, supervised or not. A mature 12 year old I'd allow to decide for himself.
You seem to be forgetting the predicament you may be putting someone else in, a few good years back a ride in the Pentlands was cut short by the need to get someone off the top after they bust their head, no helmet. So helicopter out and a day spoiled through one riders selfish indulgence and vanity?
Or how about an accident which bears more injury due to no helmet, if this is caused by 'No Helmet' the drivers life may still be badly affected regardless of it being the bike riders choice. So regardless of it being up to the individual, an accident may affect someone else's life just as much as yours.
Or kids seeing an adult riding trails and jumps with no helmet, lets say that influence leaves the helmet their parent bought at the side of a jump, and the kid hurts his head, I feel it is part of social responsibility to set an example.
Though as an example, I am sure a cyclist did die due to a truck mirror hitting his helmet, I think it was something due to the helmet directing leverage and force in a certain manner the rider died instead of suffering a fractured skull?
I would like to see newer improved tests and methods into how helmets behave in an accident, and the effect they can have regarding neck and spine injury, not to mention the way they can apply leverage and direct force at certain angles.
For instance if you fall back, many helmets put more strain and pressure on the neck forcing it even further forward, and the same applies to a face plant where some helmets can force the neck back.
You also have side impacts with ground at speed, which can effectively twist your neck and head, due to the helmet gripping into the ground where a head may miss a few inches or lose some skin.
A lot of this does not seem to be tested against or have any proven protection. I remember seeing early tests which involved an impact straight on top of the helmet,which is nothing like an abrasive sideways impact with weight and leverage at speed.
I am also sure there are no proven statistics or figures regarding injuries caused due to no helmet versus helmet or any correlated figures regarding injuries actually caused by the helmet itself during an accident.
Out of interest does anyone change their helmet due to it's age?
Personally I still believe for the majority of riders and rider environments that a decent bicycle helmet is a good and responsible habit.