Why is David worried that they won't arrive in time to save his parents?

From the text:

“We’re taking too long,” he finally said.

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It took time to prepare subjects for the operation, especially if they were older than sixteen. But Dr. Cable wouldn’t wait forever to turn David’s parents.
Every day the storm delayed them, there was a greater chance that Maddy and Az had already gone under the knife. For Shay, the perfect age for turning, the odds were even worse.