While Shashalle isn't as young.

The difference in look is very easily explained.

When she was bound, Siuan was barely into adulthood (she was nineteen, wasn't she?). Now she's around 40, with 20 years of so of slowing. Her face has matured into adulthood, while the slowing made it retain a teenage girl quality at this point(like Nynaeve). She looks different from the way she did at 16 (maybe put on a little weight and curves, her adult traits would have emerged fully since etc. so she looks like a different but still very young woman). Same thing for Amico, Leane.

Shashalle, however, has probably reached the age (still not very advanced - perhaps she looks like she's 25 or 30 so 0- some way into maturity - no longer a teenager in look, but still in her prime? I don't remember how old she actually is.) where losing the ageless look simply made her look like a slightly older version of herself when ageless. With the duress she went through post Dumai's Wells, a potential loss of weight living among the Aiel and the stress she's under now, this probably doesn't attract much attention, and sisters who notice the change probably think she just look a bit more tired than the rest of them.

The most dramatic changes in looks would occur with the fairly young and the very aged sisters, but it would not be as dramatic with the old ones as with the 40ish sisters, as even Cadsuane isn't nearly old enough to look much past her prime. In between, you're already too old to look dramatically younger, and not old enough to look dramatically older. Cadsuane, for instance, would look just above middle-aged. She wouldn't be all wrinkled, but her face would ripe a bit, and she might show some lines to go along her grey hair. It would still be a major change, as right now only her grey hair and manners belie the possibility that woman isn't 20, or 40 etc.

The "ageless" look is really just a matter of skin stretching. This has no effect at all on the rest of your body. You grey as you would normally, your eyes gain "maturity" etc. It makes 30ish sisters and Cadsuane look not much different overall. It may even have played a role in shifting the ranking from age to strength, as there's not much cosmetic difference between very old or raised a few ago, it would rattle young sisters to be ordered around by women with no physical evidence they're more experienced than you, and if the ranking worked by experience as AS, sisters would constantly be asking each other how many years they've worn the shawl - which would be terribly annoying and be very reminescent of bikering to be in charge, and this for decades and decades of being reminded verbally very often that you're too young to count just yet. The current ranking is unfair, and depressing/demotivating for the below average sisters, but strength is the one thing Aes Sedai can judge at a glance, and know immediately where they stand in a group, with no need for verbally imposing their vaster experience verbally on everyone. The other factor is that strength is quantifiable, when experience is a lot more relative and debatable, and open the Pandora's Box of competence. Right now sisters might question the system when forced to be under a stronger sister they find incompetent, but there's still no denying raw strength. Under a system based on experience, they'd question the worth of that experience, and how it relates to competence and intelligence and other qualities like clarifity of vision, leadership, etc. That's the sort of system a male organization might have, but it's not so surprising that women opted for a system that based rank on something unquestionnable by anyone. The way Aes Sedai constantly biker, it wouldn't be pretty if they based rank on something relative and debatable.