Mishnah Taharos 3:6: A deaf-mute, an shoteh, or a minor, if they were found in an alley that has an impurity in it [but there is a doubt as to whether it rendered them impure], they are assumed to be pure. But anyone fully competent is assumed to be impure. And [this is because] anyone who does not have sufficient understanding to be questioned [about his purity status], when his status is uncertain, he is [assumed to be] pure.

חֵרֵשׁ שׁוֹטֶה וְקָטָן שֶׁנִּמְצְאוּ בְמָבוֹי שֶׁיֶּשׁ בּוֹ טֻמְאָה, הֲרֵי אֵלּוּ בְחֶזְקַת טָהֳרָה. וְכָל הַפִּקֵּחַ, בְּחֶזְקַת טֻמְאָה. וְכֹל שֶׁאֵין בּוֹ דַעַת לְהִשָּׁאֵל, סְפֵקוֹ טָהוֹר:

Mishnah Taharos 3:7: A toddler found at the edge of graveyard holding roses in his hand, and the only roses [in the graveyard] are in the impure area , he is pure, for I may say: Someone else picked them and gave them to him. Similarly a donkey among the graves, his vessels are pure [since the donkey and its vessels also cannot be questioned].

תִּינוֹק שֶׁנִּמְצָא בְצַד בֵּית הַקְּבָרוֹת וְהַשּׁוֹשַׁנִּים בְּיָדוֹ, וְאֵין הַשּׁוֹשַׁנִּים אֶלָּא בִמְקוֹם הַטֻּמְאָה, טָהוֹר, שֶׁאֲנִי אוֹמֵר, אַחֵר לִקְּטָן וְנָתַן לוֹ. וְכֵן חֲמוֹר בֵּין הַקְּבָרוֹת, כֵּלָיו טְהוֹרִין: