Answer:
I start new then become old. Start clean but be one dirty. I usually start big then become little.
I am what no man ever yet did see, which never was, but always is to be. What am I?
While you were folding a letter you got me. Usually you will do whatever it takes to avoid me but now you can't help but find me.
When young, I am sweet in the sun. When middle-aged, I make you gay. When old, I am valued more than ever. What am I?
Soft and cuddly I'll pull on your heart. But take my last name and I'll tear you apart.
Almost everyone needs it, asks for it, gives it. But almost nobody takes it.