Good question. But Goldy doesn't falter in her answer.

"The immortality phenomenon didn't kick in until we Fables took refuge on Earth back in the fourteen hundreds, supposedly because this is the place where our stories are popular. In our Homelands, we all aged, albeit much more slowly, and we could be killed there. Hence why we fled from the Adversary's more powerful invading forces and eventually exiled ourselves. Wherever we were at age-wise when we came to Earth, was where we ended up sticking."