Question

what does it mean that patricia \will never know a gray hair?\

Explanation:

This is a figurative expression. Gray hair is often associated with aging. Saying Patricia "will never know a gray hair" likely means she will either die young or never experience the normal aging - related change of getting gray hair. It could also be a hyperbolic way to suggest she will always remain youthful - looking.

Answer:

It likely means she will either die young, never experience the aging - related change of getting gray hair, or always remain youthful - looking.