The gist of what Alex writes about age and death is that physical aging cannot be stopped, the body will inevitably perish no matter what, and only the mind can transcend. It's the thought of her brilliant mind being lost that scares her, not her hair turning grey, if what she says to Natalia in the
final cutscene of Barry's third chapter is to be believed. Regarding Natalia as the new vessel, there's a file saying that her age is a problem that time will take care of. So Alex doesn't actually want to be a little girl, it is even considered a "problem", but one that will eventually solve itself, and keep in mind that she considered older (even male) vessels as well. Christmas Girl wasn't chosen because she was the youngest, but because she had the strongest mind. But of course, the younger the vessel, the longer she can use it before she has to start looking for a new one, so youth does have its obvious advantages when your goal is to live forever.
As for the cloning thing, well, that's something RE6 introduced years later (chronologically; I do realise Rev2 came out after 6, but is set earlier), and while Simmons may or may not have somehow gotten his hands on Alex's research to override Carla's original mind with Ada's, I don't think Alex had the C-virus-related cloning technology yet. That's not what she was researching anyway. And even if she did create a new identical copy of herself from the ground up using some other method, that copy would likely come with the same afflictions as the original body, which defeats the purpose.
Besides, I've been thinking about opening a new thread about hopes, wishes, and theories concerning a possible Revelations 3. Would anyone be interested in that?