Video: Levinson's Stages of Adult Development Theory. the time Doris left for college (), was a key beginning stage in her adult development. Your child is growing and changing by the day. From the time of her birth through adolescence, your little one moves through a steady progression of milestones that include every area of development. Intellectual Development. a fifteen-year-old girl may physically resemble a young adult but she may still act very much like a child since it isn’t until late.