Answer:
When young I am sweet in the sun. When middle-aged I make you gay. When old I am valued more than ever.
I am a tale in childrens' minds. Though an illusion, I occur every night. I give them a fantasy, I give them a fright. What am I?
I fill a room but take no space. What am I?
I am gentle enough to soothe your skin, light enough to fly in the sky, strong enough to crack rocks. What am I?
I am flora, not fauna. I am foliage, not trees. I am shrubbery, not grass. What am I?
No matter how little or how much you use me, you change me every month.