Htydjtk

At a young age, Nobody Owens was given the Freedom of the Graveyard, which gave him the ability to pass through solid objects, talk to the ghosts that lived in the graveyard, etc. He makes good use of these abilities throughout the book, especially as he is notably being raised by ghosts.

However, towards the end of the book, we see that this is starting to wear off:

Bod walked over to the pointed stone—tall, carved with upside-down torches, and he waited, but saw no one. He called to Alonso Jones, even knocked on the side of the stone, but there was no response. Bod leaned down, to push his head into the grave and call his friend, but instead of his head slipping though the solid matter like a shadow passing through a deeper shadow, his head met the ground with a hard and painful thump. He called again, but saw nothing and no one, and, carefully, he made his way out of the tangle of greenery and of grey stones and back to the path.
_{Chapter 8: "Leavings and Partings"}

And we see he's around 15 years old by then:

But Mother Slaughter was pursing her lips and tilting her head. Bright old eyes scrutinized Bod from beneath her bonnet. “I called you boy, didn’t I? But time passes in the blink of an eye, and it’s a young man you are now, isn’t it? How old are you?”

“About fifteen, I think. Though I still feel the same as I always did,” Bod said, but Mother Slaughter interrupted, “And I still feels like I done when I was a tiny slip of a thing, making daisy chains in the old pasture. You’re always you, and that don’t change, and you’re always changing, and there’s nothing you can do about it.”
_{Chapter 8: "Leavings and Partings"}

After that, Silas tells him that he can't stay around the graveyard, and that he has to go leave and live his own life.

But why did these abilities start to fade when he reached the age of "about 15"? Why would the Freedom wear off then? The Freedom hadn't worn off for Silas, even though he'd had it for more than 15 years.

There are two major theories I've encountered:

1. It's a parallel to The Jungle Book where Mowgli, having rejoined the world of Man, either cannot or will not speak with the animals anymore.


2. The Freedom of the Graveyard was given to Bod to protect him against the Jacks of all Trades. With them defeated, there was no need to continue to grant him that ability.

In contrast, Silas is already a creature of the Graveyard, and remained so.