They (the docs, pediatricians, etc.) would've noticed pretty early on whether she was missing it or not. When it breaks, it still leaves a ring of tissue, and can tell if there was tearing if recent enough. In this case, Amy was considered quite normal until that night, and showed no signs of breakage- There were no signs at that point whether she had a hymen at all, and certainly no tissue stretching and tearing, so she clearly showed no signs of being raped-- No-one could figure out how she could have a spontaneous orgasm at 5.
Yes, 5 is a bit young, but I've talked with some girls (in their teens by that point) who were already exploring themselves by the time they were 5 or 6, so it's not entirely out of the realm of possibility. The thing, though, is, before that event, she was about as clueless as any young girl her age, yet afterwards, she knew what sex and sexuality was, and she understood the pleasure she was feeling down there-- Yet no-one ever told her anything or ever touched her inappropriately, so how could she know? Her own answer is that she just knows, and doesn't know why or how she knew then. She DID understand why many were uncomfortable with a child knowing enough about sex, and knew how it felt, and knew it was a private thing-- And kept it that way- Only her mother knew, but she did respect the fact even her mother was uncomfortable about it. (Her mum did give Amy her first sex toy, by the time she was six-- A miniature version of this toy (link). )
No, age progression is about the same for the furs of her world as for the humans of ours. Only the size difference, depending on species, is different-- Mice normally grow to 4.5-5' tall, whereas Amy is 3'11". However, the second-tallest and second-shortest living furs of her Earth are, in fact, mice- 7'9" (female) and 2'9" (male), respectively.
I have been planning, off-and-on, to write a story about Amy's first times, with both a girl, and a few days later, a boy, at the age of 9, when she just couldn't stand the pressure inside, and wanted to have sex with her best friend, whom she had to then secretly had a crush on. (The boy- a rabbit- was another matter- Amy was curious, simple as that!) Been unsure whether to go through with it or not.
Ironic to say, Amy tries her best not to let her sexual life rule her life, period, and doesn't even let it dominate. She was, however, the most-read girl on the subject, at that age.
Then, there's the matter of her near-obsession with panties... ;)
(Near?? ;) )