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It's hard to say whether a gifted 14-year-old is "too old" for this book or not - it's right on the borderline. (It would go back to being just fine for a 25-year-old, BTW - but for that difficult "prickly" age, I'm not sure.)
On the other hand, asking a kiddo if the author seems to have it right... possibly saying that parts of the book remind the parent of how things felt for them at that age... asking the kiddo for their feedback on the book... may be an effective way to spark interest?

Any kiddo suffering from worry or anxiety may benefit from ideas presented in the book:
- the bit of scientific background provided,
- knowing they are not alone,
- knowing what can be done (what they can control),
- seeing a number of examples in the third person (so they do not personally feel pointed out)
... and possibly most of all by having a name for it which they could use to talk to their parents, whether "The Worry Monster is plaguing me again!" or "my amygdala is being activated"... anything which affords them the dignity of keeping their situation from being unspeakable or resulting in accusatory name calling ("You're such a worry-wart.")

The book is non-threatening to a teen; A reader is not likely to feel less-than after reading in the introduction that the author has experienced a bit of worry as well.

Books are good that way... a person can read a lot or a little, whenever they are ready.