Why is Dorian so afraid of aging?

Over the course of the story, Dorian comes to fear aging. He believes Lord Henry when he tells him that life won't be worth living.... that excitement and experience move beyond one's grasp.

You have a wonderfully beautiful face, Mr. Gray. Don't frown. You have. And beauty is a form of genius -- is higher, indeed, than genius, as it needs no explanation. It is of the great facts of the world, like sunlight, or spring-time, or the reflection in dark waters of that silver shell we call the moon. It cannot be questioned. It has its divine right of sovereignty. It makes princes of those who have it. You smile? Ah! when you have lost it you won't smile. . . . People say sometimes that beauty is only superficial. That may be so, but at least it is not so superficial as thought is. To me, beauty is the wonder of wonders. It is only shallow people who do not judge by appearances. The true mystery of the world is the visible, not the invisible. . . . Yes, Mr. Gray, the gods have been good to you. But what the gods give they quickly take away. You have only a few years in which to live really, perfectly, and fully. When your youth goes, your beauty will go with it, and then you will suddenly discover that there are no triumphs left for you, or have to content yourself with those mean triumphs that the memory of your past will make more bitter than defeats.