"Older people are people, too. Even though they are grey-haired and wrinkled now, I know that they used to be my age, and that one day I (probably) will be their age."
Yup... & ya know what else? You will always be someone's child...
I think that the key to staying young at heart is to not define yourself too rigidly. Also, don't necessarily accept the "definition" that others may have of you. Continue to learn new things, challenge yourself & and to grow :)