"Semi-centenarian, a person of 50 years of age; semi-centenary, the fiftieth anniversary; so semi-centennial a."
As the language evolves, perhaps we'll settle down to semicentarian. We do need to get rid of hyphens when we can.
Update, Feb. 17. How about Demicentarian? Semidemicentarian (for 25 year olds)? Presemicentarian (for 49 year olds)?
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