What I would tell my boys:
Be careful about thinking the grass is greener on the other side of the fence. If you jump over the fence, you may land in that what makes it greener. Perhaps that fence is there for your benefit.
My grandma's response when I would say I couldn't wait to be a grown up:
"Enjoy your time now. You will be grown up for a long, long time."
And the one from my son:
"Would you like some cheese and crackers with that whine?"
Then, when an adult does something stupid..."only one letter away from 'adolt' "