I fail to see how measuring adults' brains tells us what they were like at birth. Can't brains change? I can recall recently reading about how neurons actually

I fail to see how measuring adults' brains tells us what they were like at birth. Can't brains change? I can recall recently reading about how neurons actually can be created anew, in contradiction with ancient established fact. Can the structures of brains not change as well? Perhaps the brains of these adults have shifted along with their personalities.