Siuan's commentary on ages for Sitters coupled with the various hierarchy things we've seen tends to make me think she is correct. Given all the rules in the Hall we've seen that have to do with Sitter Age I tend to buy into certain elements being spot on for Ajah Head selection as well as Amyrlin selection.

Certainly every woman we've seen (and know the relative strength of) who is a Sitter or Ajah Head or other high level AS has been either VERY strong (by AS standards) in the Power, or very experienced and in most cases BOTH. The Salidar Six are an example of this ... they are almost certainly among the strongest women from each Ajah, strongest of any Ajah in all likelihood. Then take a look at the normal age Sitters in the rebel Hall ... Romanda and Lelaine are both older and very Powerful and end up as the major faction leaders. It seems natural that the hierarchy would have an effect on such things. Strength is the #1 criteria followed by training time and Age in every case ... in the Hall only length of time as a Sitter trumps that ... but realistically even Sitters defer to other Sitters with more strength, as we saw with the BA Hunters, note that Saerin is the Strongest (or at least equal to), the oldest, and has served the longest time in the Hall. Seems obvious that the Ajah each end up selecting the most experienced Sister who has sufficient Strength to not get pushed aside for such roles. Picking an Ajah Head doesn't seem much different ... why would the Blue select Cabriana as AH when they know that she has spent decades deferring to pretty much every other Blue sister? And why would they choose Siuan or Moiraine when they have women with both centuries of experience and very high strength like Anaiya or Lelaine? No way Cabriana ever gets the Ajah Head job. Siuan or Moiraine on the other hand would have just needed time to prove capability. Unless something drastic like the rebellion happens.

I suspect Myrelle was chosen Green Ajah Head for the rebels (something Elayne was SHOCKED by) for the same reasons the too young Sitters were chosen ... give her some experience prior to the actuality happening. As one of the Strongest Greens she would have been on the short list in the natural course of events. But probably not a viable contender for the role for at least another 100-150 years.

Imagine you are AS and have to vote for a new Sitter for your Ajah. Knowing that a weaker Sister would not get deference from other Sitters who would you appoint if you had to pick a Sitter ... 145 year old Sister Talene, who is Strong, or a Sister who is 215 years old but as weak as Hattori? This is about Politics, not 100% about ability, a weak Sitter would have a lot of issues commanding the respect of other Sitters and would end up losing ground for the Ajah. It's all about who will be able to sway Sitters from other Ajah to her way of thinking more easily. Same can be said for Ajah Heads. The Heads we've seen and know the relative Strength of are all very high in the hierarchy. Galina, Eadyth, Romanda and in all probability Anaiya and/or Lelaine all stood very close to the top of AS Strength. With only Galina being young, and her position was almost certainly attended due to BA maneuvering.

In real world terms, who do you pick for the team captain? The new guy with potential but has only been playing for 5 years, or the guy who has 10 championship rings and has been playing for 20 years? Sure you don't pick the veteran if he has no leadership skills, but if he does he's the natural selection.

AS are creatures of habit and rules and control. We only see them vary from their traditions under extreme circumstances. Siuan is chosen as Amyrlin because there had been a succession of short term Amyrlins and the Tower was reeling from the Troubles and untimely deaths of senior Sisters. She naturally appointed other young women as part of her immediate circle, but since both are extremely Stong for AS it is easy to let pass ... seems natural for her to select Sheriam and Leane since they were among her closest friends from Novice/Accepted days. Egwene, as we all know was chosen because they thought to set up a puppet. Yet they chose someone they thought could eventually (due to her Strength) become a true Amyrlin, Siuan played on this deliberately when she chose Egwene. They would not have selected a weak woman ... they had plenty to choose from in Salidar.

Elaida was a creation of the BA and when she was forced into a situation to replace Silviana she needed to keep the Red on her side, find someone she felt she could trust (i.e. a contemporary) and probably had some level of BA influence on that selection as well. If I were to guess at Silviana and Tianna's ages I'd assume they were probably younger than what a Sitter would be, but experienced enough to have held other administrative roles. MoN seems to be the type of role you could put a woman with potential if you needed to ... since they don't actually participate in Tower policy you are safe if you end up with some one like Tianna who gets overwhelmed.