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Silicon Valley's talent wars are going younger. Bay Area tech companies, already in a fierce fight for full-time hires, are now also battling to woo summer interns. Technology giants like Google Inc. have been expanding their summer-intern programs, while smaller tech companies are ramping up theirs in response—sometimes even luring candidates away from college. Dropbox Inc. plans to hire 30 engineering interns for next summer, up from nine this year, says engineering manager Rian Hunter, who adds the company wants interns to comprise one-third of its engineering team. The San Francisco-based file-sharing company this year dispatched its entire engineering team to recruit at more than a dozen colleges, up from just five schools last year, schmoozing recruits over dinners and through technical talks on subjects like how Dropbox reduces the amount of memory its desktop client uses.   Interns allow you to "try before you buy," says Bump Technologies Inc. Chief Execut...