Spoiled children can grow up, but it's a steep uphill battle. Also, such individuals would have to reach a point of wanting to change. Spoiled children never learned the importance of delaying gratification, thus they assume life should be easy and "things" should come to them. They reject the notion that anything worth getting takes work and discipline; consequently, they suffer from entitlement issues and tend to make lousy friends and partners.
Some spoiling of a child is natural, but too much really ruins the child and future "adult." I wish more parents knew this. On the other hand, being too strict is also problematic. Good parenting requires balance. One size does not fit all. Each child should be respected for their individuality.