18-year-olds have legal majority, but talk to one for an hour and you'll see most are not 'adult'. 500 years ago a 16 year old could be a man, contributing as a full fledged adult to his community. Today it's 25 if you're lucky.
From what I understand this isn’t actually true to much extent; 16 was still considered fairly young. Too young to run a household or be the head of the house, often times, although if it happens it would be a forced of circumstance. When Henry of the six wives married a 17 year old (by some estimates) people were disgusted/concerned about it.
You're implying that being selfish is inherently bad. Of course it's selfish. You're doing something for you. It's only a problem if it unfairly hurts someone else. How is that the case here?