The logic I’ve always gone with is that it -blooms- until his 21st year, but that doesn’t mean that it wilts as soon as he’s 21. So he could have been closer to 15 or so. And Lumiere might have exaggerated the length of the curse a bit, too.
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Keeper of Belle's Gold magic, sand dollar, cloaks, purple FTL outfit, spell scroll, library key, copy of Romeo and Juliet, and cry-muffling pillow, Rumple's doll, overcoat, and strength, and The Timeline. My spreadsheet: https://drive.google.com/file/d/0B6r8CySCCWd9R0RUNm4xR3RhMEU/view?usp=sharing