Interestingly enough, her age regression might be the thing Cy cares the least about– it's inconvenient and disorienting for him when it happens, and it's not like she regresses to an age he'd be attracted to anyway.

But you're not that far off, it's more that he enjoys the timelessness of her character (something she has in common with Lynn), this combo of stunted in some aspects and wise beyond her years in others, both due to trauma.
It's also something he can relate to (minus the trauma), and I think he really appreciates how hard Isabelle works to overcome not just her mental trauma, but the physical too (she's got serious brain damage and a chronically ill body)