There isn't some magical switch that's flipped on your 18th birthday where you suddenly be pragmatic and make rational decisions. Even into your 20s, your brain is still developing. My mother is in her 60s and addicted to facebook and tiktok. It takes significant willpower to overcome addictions for most people. The brain of an 18 year old addicted to social media is mashing the "do the feel good thing" button just the same as a 17 year old.