I don't think they are bound by the channeling potential of their new bodies. The decisive argument against Cyndane being Lanfear for Demandred was that she was weaker. If transmigration altered your strength, that would not have been a consideration, but Demandred is ready to toss out all the other evidence that points to Cyndane having been Lanfear, just because the women say she's not as strong. So clearly, their own channeling nature for their current lifetime imposes itself on the new body. Clearly, the new bodies were picked in recognition of the Forsaken's natures, to twist the knife a bit, but that would have really been stretching credibility if they went out looking for people who met the physical qualifications, AND were super-strong channelers.
Therefore, if the ability to channel can be imposed, regardless of the body's proclivities, it stands to reason that the side effects would also impose themselves. The Companion states that the longevity is an effect of the channeling, rather than a part of the ability, suggesting that drawing on the One Power has beneficial effects on the body (actually, if the age thing had not been explicitly stated, I would have assumed it would consume your physical form faster - that seems more appropriate to me, especially with the descriptions of an overwhelming external power), and thus it would affect whichever meat suit has the Power flowing through it.
In the same line of thought, how long with the 13x13 Asha'man and Aes Sedai stuck in the stedding last without channeling? What about the male Aes Sedai who retreated to the stedding? Would they have started eventually aging faster (assuming they had to first overcome the cumulative effect of past channeling)? Would their bodies have been reset to their 20s, as with severing? What about someone born with the ability who refrained from using it? What if Rand had hightailed it to a stedding after Tarwin's Gap, and spent the rest of his days discussing literature with Loial and teasing Aiel with his sword (assuming a non-Tarmon Gaidon era, of course). How long would he last? About eighty years or so, or would he get even a part of the channeling extended life? What about if someone devised a ter'angreal or a weave or something that shielded a male channeler from the taint, but it greatly reduced the amount of saidin he could wield. Would he age at the rate of his potential, or at the rate equivalent to someone with the same strength he is actually using? I'm going to assume, for the sake of argument, he achieves his full potential, because he is still drawing on the Source, using the muscles, as it were, he just can't use as much of what he can draw on.
And assuming we had an answer one way or another about the stedding, would the same thing happen to a male channeler who moved to Far Madding? What about someone who was simply shielded, and had it knotted? How would that person age?
Based on my Forsaken answer, I'm going to assume it would be a normal lifespan for all those who were cut off from the Source in any way, to be rectified once the regained access to the Power, a la RJ's answers about Sharina.
And what did that mean about Siuan and Leane reverting to childbearing age? They weren't all that far past it. Leane was 47 when stilled, and Siuan 42. Forty two is a bit extreme, but not unheard of. If there was ANY preservative effect going on, you'd think they'd have still been in good shape, unless that's part of the Oath Rod effect.
But I'm not sure about the lifespan in a Stedding. We know that Nynaeve Slowed before she went to the Tower even though she was only sporadically using the Power so I don't know that the amount used on a daily basis really makes a difference in the lifespan once the ability has been activated. It's very possible that a channeler would live their full, extended, lifespan even if they never channeled another day in their life.
We also know from looking at Setalle Anan/Martine Janata that once cut off from the Source lifespan appears to reverts to normal, starting from the time the Channeler is cut off. According to the Companion Martine Janata was born in 902 NE, which makes her 98 by the end of the series, she burned out in 973 and as Setalle she now appears to be in her 40s and over the last 27 years clearly aged at a normal rate since her own family doesn't appear to know that she's actually nearly 100 years old. Given that she was strong for an AS (level 14) she probably didn't appear more than 20-something even though she was in her 70s when she burned out, so it seems she started aging normally from roughly age 20-25 and now seems like a normal 40-something year old woman. The Companion does point out that she will, as a side effect, always appear slightly younger than her age, but there is no reason to believe that she will live for more than a normal lifespan post burn-out.
It's also probable that some of the advanced effects of Siuan, Leane and Setalle had to do with also being severed from the Oaths which restricted their age potential in the first place, Sashalle seemed to have had physical changes as well so the Oath Rod seems to be playing a role in these effects.
Sharina, getting such a late start in Channeling is a bit different IMO, RJ did say that she will eventually revert to looking middle aged as opposed to old, but who knows how limited use of the Power would effect that
