ashes

The speaker is saying that his youth his dead.

The speaker is comparing himself to a burning pile of wood. When the speaker's life began, he was a whole pile of wood. When he was born the pile was set on fire, and now the fire has consumed all of the wood, leaving only a few glowing embers. The ashes that represent his youth are underneath the embers. Pretty soon, the fire will go out, and the the speaker will die thus turning into ashes just like the burned wood.