If she hadn't been gone for months this time, she might have missed it. Humans looked odd to her, but she had non-clones around on this base of operations to actually make a relative comparison with. One of the techs she knew well hadn't changed in his face nearly as much as Rex had.
And she could see those changes in Gregor, in Wolffe, in the handful of other shell-shocked clones they had managed to rescue from the Empire.
If she hadn't just come back from lengthy lessons in how to apply the Force for healing at levels the Jedi only dreamed of, she might have waited for the scientists Organa could bring to bear on this.
If Rex wasn't her anchor in the Light, she would have listened to his protests to stop.
If she didn't need Rex to have one good damned thing in his life, she wouldn't have pushed through as hard as she did.
If … the word was normally her bane, but Ahsoka Tano refused to let this if stand in her way. Her beloved, her mate, her chosen partner to shoulder the adversity of life, was not going to die before her because of the kriffing Kaminoans.
There were flickers of the Dark Side touching her, as this was the most selfish use of the Force she had ever done. She, who walked a middle road, knew the temptation to push too far, to bind the life-force she was renewing to her own, but she stopped short.
Once, twice even, the witches that had crafted this power had created near-unstoppable warriors with it, beings bent for evil. Ahsoka took those teachings now and crafted something better. She gave to Rex his strength, his stamina, all the things of his prime that the aging wanted to chip away from him. And then she broke his ability to age in anything approaching a normal fashion, even if that wasn't quite what she meant to do.
She loved him too much to lose him, and paid the cost willingly, passing out over him in their shared bed.