Choice Number 4: I don't care.
The research methodology in the OP is flawed for many reasons, the first of which is that it's not possible to create a control that separates cannabis, tobacco, and alcohol use from one another as variables for the effects observed.
Most of the aging population that were heavy cannabis uses for most of their lives are only now reaching the age where research can be done to determine what life-long use has accomplished.
Up until now, the claim that cannabis causes permanent damage to the brain has been continuously debunked. It does however cause short-term impairment that is reversible once stopped. I'd personally be more concerned about the effects on the lungs if you're not using a vaporisor.
"To waste, to destroy, our natural resources, to skin and exhaust the land instead of using it so as to increase its usefulness, will result in undermining in the days of our children the very prosperity which we ought by rights to hand down to them."~ Theodore Roosevelt (Message to Congress, Dec. 3, 1907)
From what I've gathered, weed doesn't outright kill braincells. It just makes you too lazy to actually use them and they eventually atrophy with disuse.
WRT early use of marijuana, the mid - late teens is a crucial period in the psychological development of the young person. If you must use this stuff, please try to defer it until you are in your twenties, or use it only very occasionally.
Every political good carried to the extreme must be productive of evil.