Judgement. The ability to hate others for perceived differences. Kids don't do that, at least not until they grow up a bit and learn it from their parents or other influences in their lives. Kids let one another live however they please.
Growing old is part of life, but the 'old' part is nothing to write home about. Live your life to the full for as long as you can, because when time starts to run out the last part of your life can be a pain, literally. Our bodies grow old but, for the most part, our minds are still younger than this shell that carries us through this world. I'm 68 (in November) but in my mind I am still 18 but this bloody body won't comply with what my 18 year old mind wants to do. Stay young, don't grow old; not that you have much to say about it lol.
As for the worst thing about becoming an adult, is losing the innocence of youth.
The worst was watching my parents becoming sick, frail and approaching death and being helpless to do anything about it. I am now at the age where my own health is compromised by age related wear and tear and my friends are in the same boat. There are so many things we'd still love to do but physical limitations prevent us from doing them which is a shame since most of us are now retired so there's plenty of free time.