The original question was about why agencies want younger girls. I base my response to that on what makeup artists tell me all the time ... they see the models' faces close up, day after day. And this is what bookers tell me too. If they are wrong, it's still the reason they prefer youger models... based on what they and the clients believe.
And I am generalizing ... of course, some faces look younger than others and those who take care of their skin can maintain youthful looks longer. Many models do not take care of their skin properly and are subjected to lots of weird things during the course of their careers ... so maybe that is another reason.
But you won't have any 70+ year old woman who looks like a 15 year old ... even if she's been out or the sun all her life ... aging still happens. More for some and less for others ... and skin care is certainly a factor.
Staying out of the sun all your life will certainly lead to "Remarkably young-looking skin well into their seventies" ... but that does not mean that aging has stopped at puberty. That comment only means that the skin is unusually yourthful for a woman of that age ... maybe she looks 50 instead of 75. Gravity, moving our faces, exposure to toxins and free radicals, and the natural wearing out of cells' abilty to regenerate as fast all contribute to aging, too. It shows on faces ...