I'd like to stimulate some discussion on this. I, personally, believe it to be ill-supported and a result of grasping at straws.

Let's see you lore-forumites persuade me otherwise, if you can. I'm quite willing to concede if more evidence can be found than the Unofficial Elder Scrolls wiki and one sentence in Morrowind.

[nevermind, could not find it]

No mention other than in the Morrowind manual. No other mention of changed sixes happen, nor does seem to make sense as argonians can get really old when single gendered. Nothing in lore what so ever other than that mention on the manual

My conclusion, bad wording on the manual, and they don't really change sixes based on age.

I agree. The trouble is the wiki. It started out as "Argonians may change gender", then the lore page had "Argonians change gender", then all pages on the race had it. I'm not one to advocate misinformation.

There was a topic about this on the UESP wiki forums. I argued against six change at all on the first page... and newcomers to the thread on page three brought it up again as though it were fact.

And it figures that Bethesda would do nothing about it. Consider this, for example: Bethesda said concretely that how many times an Argonian licks the Hist determines sixuality.

The problem is that "sixuality" has three different meanings in English: sixual preference (heterosixual/homosixual), promiscuity, and the state of possessing biological six.

Has Bethesda ever been asked for a straight answer regarding the supposed six change?

My theory is thus: Hist sap is how the Hist control the Argonian form and mind.

It's been mentioned that Argonians have a different form depending on how many times they've licked the Hist (where it's logical to think sap would be collected). Also, licking the Hist has been tied to an Argonian's gender when they lick it during some puberty ceremony (I think I read somewhere? I wish I could remember these references). This doesn't really make sense given what happens in the Oblivion Fighter's Guild questline -- the Argonians ingest Hist sap on multiple occasions, and though it temporarily drives them berserk, they don't change form. It's also been stated in the Lord of Souls that the Hist changed the Argonians to be stronger for Dagon's invasion, but I don't think goes into detail precisely how they did it.

My idea is that, when an Argonian (or maybe anyone) ingests Hist sap, it physically and mentally changes the person according to what the Hist wants. If they want you to become beefier for combat, you'll become beefier. If they want you to become a female, you'll become a female. If they want you to go mad, you'll go mad. If they later decide you should be a male, when you lick the sap again you'll change into a male. If they think you're just fine the way you are, it does nothing.

I don't know of anything affecting gender, but I did find this, from the Argonian farmer Travelling-New-Woman in Morrowind:

I've heard an alternative theory: the Argonians believe in direct reincarnation and "life-phase" is simply their name for a single lifetime. You're a woman now but when you die you might be a man the next time.

Not sure where I heard that or if there's any lore to back it up either (doubt it) but just throwing it out there.

That probably would make sense, as argonians also don't have much sense in time, and their language gives a view that there is no time.

An imperial may say he just ate some boar meat, but to the argonian, there was no past.

This is a bit of a necro but I've been very curious about the "Argonian gender-change" controversy for some time (and I've nothing better to do than troll the forums right now while I wait for midnight). I read the aforementioned UESP forum topic in its entirety and did a lot of lore digging myself to try and determine my own view on the matter, but to no avail.

Honestly, the prospect of Argonian gender change had been my one determining turn-off to Argonians before I realized there was some contention as to the legitimacy of it. As had been said in the topic, a person's gender (and perhaps I'm just being overly-influenced by Western notions of gender distinction) determines a great deal about their life experiences and views. It feels almost uncomfortable for me, in an RP sense, to visualize myself in my character's shoes with the ever-present knowledge that one day I, as a female Argonian, perhaps even with husband and child, could find myself a male with (if the stats are any indication) a more aggressive disposition and a fraction of my formal intellect; in essence, a different person entirely.

I think it's a cool and weird part of Argonian culture that we just never see in the games. None of the games have taken place in Black Marsh so it's never been convenient for an NPC to go home and lick the sap.

Argonian life-phases reformatted. Software reinstallation, but the Hist processors remain the same.

I could only read the first post with this voice. http://www.youtube.com/watch?v=zP2m95JAD4g#t=1m44s.

What qrsComplex said.