“When you can stop you don't want to, and when you want to stop, you can't…”
Source: Candy
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Luke Davies 6
Australian writer 1962Related quotes

“You don't stop laughing when you grow old, you grow old when you stop laughing.”
Variant: We don't stop playing because we grow old; we grow old because we stop playing.

“Why is it when you want the ground to swallow you whole, it decides to stop trying? -Ethon”
Source: No Mercy

“Don't let things you can't do stop you from doing things you can.”

The correct attribution http://www.snopes.com/politics/quotes/carterpoor.asp is comedian John Fugelsang, on the TV program Viewpoint (29 May 2013)
Misattributed

“You can say what you want about me
Try to do what you want to me
But you can not stop me.”
Paid My Dues
Freak of Nature (2001)

http://groups.yahoo.com/group/cerebus/message/100634