Because Adam's power was keeping him alive for hundreds of years (since feudal Japan anyway) and without it, his cells reverted to their original age.... dust.

Body cells don't last the entire lifespan, they are being replaced over and over. Peter should actually die (after losing the power) from all the wounds he received and healed from, if that's how it works...

I don't see why. The cells weren't being held together by his powers. The healing power just caused them to close up like they would naturally, only really, really fast. With the power gone, if he got hurt now, he'd just heal at a normal rate of speed, but there's no reason old wounds already healed would reopen. Adam's cells though, were only being alive because of the power cause no one lives for centuries naturally.

In fact, this non-power time might be when Peter gets the cut across his face that leads to that scar Future Peter had. I always wondered why Future Peter had that big whopping face scar. Must mean he'll get his powers back somehow too.

Reopening wounds sounds kinda absurd of course, but this regenerative process seems just like the natural cell replacement process to me, so Adam shouldn't go ashy either.
But maybe it just works differently, like you are saying his body cells don't get replaced like in a normal person, but never die naturally. It's still a shaky theory, but it would explain why he disintegrated.

Going for science with this stuff will hurt, better leave it as a fairy tale it is.


Actually, they are struggling for that future never coming true, so...
But yeah, I bet he'll get the scar and then his powers back.
I'm not sure how though, his father must have taken his power-absorbing ability too, cause otherwise Peter would just get all his powers back right away at the spot. Unless he will, just after the cliffhanger and receiving the scar. But If he can't absorb anymore, how will he get abilities later?

There's still that formula. The one that works. Maybe Peter gets a shot of it and it reactivates his birth power... which I guess he inherited from his dad. His dad seems to have the ability to steal powers, and both Peter and Sylar have similar powers (Peter just needing to be around someone with a power to use it and Sylar needing to understand the power to use it).

Or maybe if the old man dies somehow, the powers he stole will revert to their original people. Assuming they're alive of course.

Who knows?

Well, Peter seemed to push away as quickly as he could and Arthur didn't seem much up for killing him in the first place. It may be that Arthur's power doesn't just drain the powers, but also energy from the body and its cells and he can control how much he wants to take.

Well, Peter was around Adam long enough to have adopted his immortality/regeneration power. So he was growing older, but not actually feeling the effects... when his powers were drained, all that aging hit him (however minor). It's the same reason Adam turned to dust, as he was hundreds of years old.

Also, one has to wonder why Peter never adopted Sylar's power by just being close to him.

And finally, KILL CLAIRE OFF ALREADY.

He prolly did have it. But that didn't mean he knew how to use it. And the thing with Sylar's power is that he has to understand a power to get it to work. That's why he has to peek inside folks' heads. Once Future Sylar explained to Peter how to use it, he could use it.