Answer:
When young I am sweet in the sun. When middle-aged I make you gay. When old I am valued more than ever.
Four fingers and a thumb, yet flesh and blood I have none. What am I?
My snake-like body cracks and burns. I cause pain but pleasure too. What am I?
I don't live within a house, nor do I live without. Most will use me when they come in, and again when they go out. What am I?
Lovely and round, I shine with pale light, Grown in the darkness, a ladyβs delight.
No matter how little or how much you use me, you change me every month.