Why do you have to be over eighteen to have your teeth whitened/bleached with an at-home kit?

i really want to have snow white teeth and they have tooth whitening kits on sale but they all say on them you have to be over eighteen. why? can it harm your teeth? i'm not gonna get it done if it does but how does it damage them? i'd just like to know thanks.
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I think because it is harsh on your teeth and they don't want you overusing it and damaging them.
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