It's not a zero sum game, high youth unemployment is not due to lack of jobs, it is skills mismatch.  Most jobs today require certain skill sets and if you don't have it you don't get hired.  Also the youth generally take the lower end jobs while those retiring are higher up as you gain from experience.  Japan has the largest senior population in the world and a similar retirement age yet they've had stagnant growth since 1998.  More seniors in the work force means more growth and this will open up more opportunities for youth.  When one retires they make less money so they spend less creating fewer jobs.