What age would you yet your kid start wear bikinis underwear?

i love the boyshort undies and Bikinis they are so comfortable got 8 pair of Boyshorts undies at rue21 for 99 cents each and then i was at walmart saw hannah montana bikini Panty set for one dollar so in all im wounding what age did you let your kid wear bikinis underwear or boy shorts ? im Age 22 i find boy shorts undies comfortable more then boys undies !