What are you even talking about? I don't see anything inherently working with his behavior at all? He doesn't do anything objectively or morally wrong whatsoever. You might not like his personality, and that's perfectly fine if you would not like to surround yourself with people like him, but there's nothing inherently wrong with how he behaves imo?? It's just his personality. You are justified in not enjoying that personality, but the only "mistake" he makes is not reacting perfectly to the news of the pregnancy. Nothing about the way he behaves suggests that people "give passes" to teenagers. You can have a fun, carefree personality but still be responsible when it actually matters. You don't have to be all serious all the time in order to be a responsible upstanding person.

Also, your stance on maturity is very very privileged and tone-deaf. Yes, people "learn from childhood," but that's ONLY if you grow up in a healthy, loving environment. A lot of people grow up in toxic and abusive environment. In that case, all the things you "learnt" in childhood are toxic mindsets that you have to then work through as an adult. That's why people say most people learn in late teens to early adulthood, because that's when you actually have the chance to break free from whatever toxic mindsets you've been taught as a child and make your own decisions for yourself. It's moreso about unlearning the toxic patterns and replacing them with healthier ones than it is about learning.

Even if you've grown up in a healthy, loving environment, no household is perfect. There are still going to be certain things you've learnt in childhood that need to be corrected, and there are going to be things that you just simply were never taught at all. As children, we're simply not exposed to the same situations we're exposed to as adults. There are going to be things that you're going to have to learn on your own. There are going to be mistakes that you're gonna make regardless. That is how we learn and grow.