[Pitchwife]

Thanks, Rumil! Here's my explanation:

I sleep and wait, I wait and sleep,
here within the shadows deep.
I watch the stars turn overhead,
t'ween branches dark and leaves unshed.
- Well, he must have sat there for quite a while (centuries, millennia?) beneath the black trees of the Dimholt.
The old grey stone is at my back,
- i.e. the Door to the Paths of the Dead
my time is long but life I lack.
- spending several centuries in suspended animation, it would seem.
Still tall am I, of noble mien,
but old and grey and weak I deem.
- 'tall and kingly he had been, but now he was withered as an old stone.'
Years from now my wait will end,
- when the Rohirrim come to Harrowdale,
and I will words of wisdom lend,
to carefree folk with open faces,
- i.e. Bregor and Baldor,
that dare to seek forbidden places.
- i.e. the Paths of the Dead.
When I've said my say, well that's my time,
- because he died after saying his say
but it's not for them, or for their line.
- as Baldor found out at the cost of his life. It was for Aragorn, centuries later again.

Great riddle, by the way! I've always found this one of the most intriguing passages in LotR - we never get to know who the old man was, how long he had been sitting there or what he was doing there in the first place. About as much an enigma (although in a different way) as the secret identity of Tom Bombadil!

And honestly, I didn't really 'squeeze in' - I guessed it more or less on first sight 2 or 3 days ago but hesitated to post the answer because I didn't (and still don't) have any idea for a new riddle. So bear with me, everybody, this is going to need some time.
(Morsul, sorry to frustrate you! I'd actually like to let you take the thread, if you've got sth up your sleeve, but my wife says that's 'chickening out', so you'll just have to wait.)