Fifteens rhymes with:
canteens, christine's, cortines, eighteen's, eighteens, fourteens, martines, preteens, routines, sixteen's, sixteens, stiens, teens
Fifteens sounds like:
fading, fadness, fathoms, fattening, feeding, feedings, fetting, feuding, fidenas, fiefdoms, fitness, fitting, fittings, footing, footings, fthenakis, futons