Why do some people never grow up emotionally and psychologically?

I grew up when I was 13 after being diagnosed with a life long chronic condition .... as do many kids when they are diagnosed with it. yet I look around at people in my generation ( I am 24) and they are so immature... as are people who are in their 40's, 50's etc and show no signs of growing up... why can't some people let go of being immature? I fear for the future