We're to assume aging is just like any other physical affliction, which healers like Claire and Adam can just shrug off.
Stealing Adam's power causes the effects of aging to finally show on him.
Wouldn't stealing Peter's power cause the effects of his head-impaling to finally catch up with him? Why wouldn't he die just like Adam did?
I've been assuming that Adam and Claire's regenerative powers work differently, in line with the "multiple paths to the same end" philosophy of having three power-vampires (Arthur, Peter, Sylar), three will-controllers (Eden, Matt, the Puppetmaster) etc who do generally the same thing in different ways. So maybe Claire has a permanent-cure power but Adam only had a temporary-cure, but once that conferred anti-aging, which I don't know that Claire has. Peter should have had both, and when he inevitably gets his sponging power back, he'll get the Claire-style power the next time he encounters her.
The writers do seem clueless about Pasdar's charisma.
They're not clueless about the need to get his shirt off.
They need to set Nathan up better as a leader of the metahumans. He's obviously got it in him, and he will be the good-guy leader. Sylar will take over from Daddy as the bad-guy leader.
Random thought: does Arthur realize he has a third son? If Gabriel and Peter were twins, could Angela have hidden the fact that she gave birth to two boys, and sent one away to try to keep him out of Arthur's clutches? That might be a good explanation for him being adopted out to just any old person. She wanted him to have no connection to the Company.
Then again, you're giving more thought to it than the writers are.
Occupational hazard around here.
As much as I'm enjoying this season so far, it's a convoluted train wreck of a story where half the things in it don't make the least bit of sense.
Even the stuff that does make sense is convoluted to the point where it can be annoying, eg, the entire rationale for Sylar and why he is doing anything at any given time. You gotta enjoy this show for the plot twists and the characters so that you're having too much fun watching to start coming up with complaints until after the episode is over.