what does ''teenage tool for spurning'' mean?
For a fun-loving free spirit, this Zeerust dump I’d never heard of sounded like the arse-end of nowhere, about as far from family, friends, the ocean and modernity as I imagined possible. Perhaps I’d been a teenage tool for spurning the possible Navy deal.

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Approximately:
 Perhaps I’d been a teenage idiot for rejecting the possible Navy deal.

Tool is slang here and can have a variety of meanings and depends on context.
Spurn can mean to reject with disdain

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Thanks for your help!

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teenage fool is almost certainly the intended wording