My step brother is like this and he's in his mid 30s. He has a personality disorder diagnosis and really struggles emotionally. He's had lots of input from mental health services but it hasn't made much difference in the long term.
I definitely think how he was parented has a lot to do with how he is now. His relationship with his mum has been very volatile since he was young. She used to deliberately manipulate his emotions in a really horrible way in order to disrupt his relationship with his dad - telling him his dad had abandoned them, dad didn't care about him anymore, he (son) was everything to her etc, etc. She would also dump him with his dad at short notice to go out partying and then be uncontactable. He would get in a huge panic thinking that something had happened to her. It was really upsetting and basically emotionally abusive. She needed him to be unhappy to prove to her ex the harm he had caused them by ending their relationship.
Meanwhile, my step dad's idea of parenting was basically to do/get whatever his son said he wanted - lots of games consoles and new toys, vast amounts of junk food, but no thought to helping his son build friendships, develop emotional and social skills or acknowledge and process the genuine emotional stress he was under from being the child in the middle of a horribly messy separation. He couldn't acknowledge the guilt he had about the separation, so tried to buy his son's love and happiness through totally unboundaried short-term parenting.
Meanwhile, my step brother started to struggle more and more socially - falling out with peers at school/college and struggling to make friends. Instead of helping him repair relationships and develop skills and independence, he was encouraged to believe that he was special, other people were horrible and that he needed to be protected from the world. I feel rather unsympathetic writing that, because he did have genuine difficulties as a teen and child that warranted additional support. But instead of helping him to gain independence, everything seemed to cement his view of himself as different, both in a grandiose and 'special' way and as someone unable to cope with a big scary adult world.
As a result, he's now an adult who has never acquired any of the practical skills needed for an independent life (cooking, budgeting, cleaning, some personal hygiene issues). And has never developed the emotional skills needed to build supportive relationships or persevere with developing the practical skills he lacks. It's like he's been fossilised as a lonely, angry teen for more than half his life. It's really grim and quite sad.