First, the science: Accessing the OP gives channelers an extremely long life-span. When one is bound, it diminishes that longevity and causes one to look "sorta-ageless." While we don't know all the details behind it, it's clear that a channler's access to the OP must be affected in some way - perhaps it's like a barrier or a filter.

Well, that barrier/filter/whatever interferes with the way that the OP "rejuvenates" or "maintains" a channelers' youthfulness. Since the offending channeler has been bound - through and by their connection to the thing that keeps them young - it makes sense that it changes the way in which their "youthfulness" is expressed.

Thematically, though, is another matter. Now, a lot of WoT's world can be fit into Christian mythology. In the Bible, the first people - Adam, Eve, and several generations of their descendants - had absurdly long lives. If you're not familiar, early Biblical people were living to the age of 500 years regularly, many generations after Cain and Abel. The presumption - which I'm voicing for the first time, so I may be wrong - is that humans were given long/eternal lives by the Christian God; the reduction in life span was caused by humankind's growing distance from that God.

God made Adam and Eve perfect, with no disease or struggle to harvest crops, or birth pains. When they turned their backs on God, they knew physical pain and toil. As the generations continued to pass, those humans were even further removed from God, and the long lives that he granted to his earliest creations...

Thematically, this matches with the WoT neatly enough: channelers that were more "in harmony" with the Pattern (e.g., not hurting or killing others) got to live an amazingly long life. Think about those two AS who founded Rhuidean - the suggestion is that they may have been actual AoL Aes Sedai, even though enough time had passed for the Aiel to become total badasses. Actually, one of Semi's POVs expressed the idea that being "bound" would actually give her a life-span.

Those channelers who were not "harmonious" were in fact going against the Pattern. When they were officially "bound" from doing things that were bad, the result of that limitation placed on them would be a shorter life.

Finally, it should also be noted: someone like Semirhage would probably only get one "binding" - something to keep her from harming others for her own pleasure. RJ went on record as saying that one Oath would not produce the ageless effect, but that 3 Oaths would. This means that, once again, the Randland folks are using an AoL object for something other than its intended purpose, and that the result would be something that was uncommon in the AoL.

If you look back over the nature of that imaginary time-period, it seems like folks may not have had several crime-level flaws at once. People's flaws seemed relatively simple, or prone to be one vice, as opposed to many. They were rather vanilla until the DO showed up... So some guy might've drugged women to get them to have sex, but he wasn't also necessarily a thief, arsonist, murderer, AND a forger of checks. That would require 5 Oaths on the Rod.

Someone like Baalthamel wouldn't need 3 restrictions placed by a binder - maybe just one. Or two, given the way he went after Shaitan was free. It's quite possible that no one in the AoL ever received more than one Oath. This makes the ageless look of the current Randland into a total accident. Which is where I sit on the matter whether we talk science, themes, or what have you.

That really took a long time to write out properly...