Life would be boring if it was forever.
Who sais we have to grow up?
Life is beautiful if we didn't have the limits we would waste it take every day as a gift and spend it as so
We age due to cell damage, Somatic DNA damage, and Cross linking(DNA damage) You can also google Endocrine Theory and Immunological Theory.
What is better - to be born good, or to overcome your evil nature through great effort? - Paarthurnax
If everybody was immature this world would be a mess. We grow up to mature, get knowledge and get wise and pass it done to the younger generation
Being a kid for a whole life is not a good option.