That's already illegal, and was illegal when it happened. Not sure what your point is?
And trying to take guns away from the lawful is like trying cars away from non drunk drivers cuz drunks drive cars.
Cars are licensed, insured, and highly regulated. Do the same thing and we'll be happier than we are now.
Why is it that you need to be over a certain age to arm yourself? That's not specified in the constitution. If that restriction is reasonable then perhaps more restrictions can be added, such as requiring annual mental health checks.
And yes, everything from a rock to a hydrogen bomb should be protected.