In Oblivion, as you move the aging dial from youngest to oldest while creating a male Argonian character, you notice that some scales turn green in what I'm guessing is their mid 20s (Or however that corresponds to Argonian aging). Does that mean (Looking at the fact that humans are most ripe for reproduction in their 20s) that the green scales are more attractive? and perhaps the red scales that come up in later ages signify old age like wrinkles?

Would that mean in stead of "anti-aging" and wrinkle preventing ointment, Argonians sell green scale dye?

I have noticed this as well and believe it is a sign of aging, there isn't any evidence to prove this though.

I'm not sure about green dye ^_^

I'm not sure the aging of Argonians in Oblivion is meant to indicate actual age (as opposed to just changing coloration). I do recall the Infernal City stating that as Argonians age their scales turn translucent.

There's also a change in markings on the face, with some appearing at what I presume is advlthood and disappearing in old age, perhaps indicators of sixual maturity.

Males seem more commonly to be reddish in colour (Amusei, Hides-His-Heart, Grey Throat, Weebum-Na, Usheeja) and females green (Deetsan, Seed-Neeus,Kud-Ei, Druja). This is most noticble in Ocheeva and Teinaava - as they are twins we can presume their tribal identity or similar category to be the same, yet their coloration is different. However, this might just be selective memory on my part...

I think you've got a point, females do seem to have more green colourations and males have more red. When the age slider is far to the left there is purple on the face, after playing the game for a while my male Argonian lost the purple and green and now has red and grey scales.

Twins among ovoviparous (egg-laying) creatures are a bit more common, bear in mind, as they tend to create more offspring in one sitting but at sparser intervals. This isn't always true, mind, but you can generalize. So, chances are likely that if you share parents, then you are twins. Oh but wait, aren't Argonians both ovoviparous and viviparous (bear live young) when they need to be, thanks to the Hist?

The markings could be sixual characteristics indicating fertility and post-fertility. But, the green could also be a common reaction to biological changes that Argonians earmarked for colder or dryer clines undergo thanks to the Hist mucking about with their biology. Don't have an alternate explanation for the red/translucence in later ages.

As a color, green isn't used that often to indicate fertility in RL nature on its own as the creature's own habitat would tend to be green and so it would be used as a camouflage but not as a "hey look at me, I'm so genetically fit that I don't need to blend into my surroundings." That is, admittedly, though a Darwinian perspective and not one based on Genetically Engineering Trees, so take it with as much salt as you want.

Unless you're using a mod, your character does not age.

Although according to the narration in the same passage, it seems Argonians don't often live to be old enough for that to happen.