Before Matan Torah (children of Israel received G-ds Torah at mount sinai) and in the future after Moshiach comes the age of accountability was/will be at 100 yeas old as Rashi Bereishis 5,32 in the name of the Midrash Rabba quotes from Yishaya 65,20:
אָמַר רַבִּי יוּדָן: מַה טַּעַם כָּל הַדּוֹרוֹת הוֹלִידוּ לְמֵאָה שָׁנָה וְזֶה לַחֲמֵשׁ מֵאוֹת? אָמַר הַקָּדוֹשׁ בָּרוּךְ הוּא: אִם רְשָׁעִים הֵם יֹאבְדוּ בַמַּיִם וְרַע לְצַדִּיק זֶה, וְאִם צַדִּיקִים הֵם אַטְרִיחַ עָלָיו לַעֲשׂוֹת תֵּיבוֹת הַרְבֵּה. כָּבַשׁ אֶת מַעְיָינוֹ וְלֹא הוֹלִיד עַד חֲמֵשׁ מֵאוֹת שָׁנָה, כְּדֵי שֶׁלֹּא יְהֵא יֶפֶת הַגָּדוֹל שֶׁבְּבָנָיו רָאוּי לְעֹנָשִׁין לִפְנֵי הַמַּבּוּל. דִּכְתִיב (ישעיהו סה,כ): "כִּי הַנַּעַר בֶּן מֵאָה שָׁנָה יָמוּת" – רָאוּי לְעֹנֶשׁ לֶעָתִיד; וְכֵן לִפְנֵי מַתַּן תּוֹרָה.
Why did Noah wait till 500 years old to give birth? So that he wouldn't need to build many arks to accommodate many decedents had they been righteous, and so that they would not have all died had they been wicked (they had free will). So when Noah gave birth to Jepheth his oldest son at 500, Jepheth had not yet reached 100 years old the age of accountability before the flood by the time Noah built the ark at 600. as Stated in Isiah 65,20:"For a young lad at 100 years shall die (i.e be accountable to liability of death for his deeds)" this is referring to the days of Messiah and before the Giving of the Torah.
Note: This age of 100 is with regards to accountability to punishment from G-d which nowadays is the age of 20 (see Numbers 1,2), however the age of physical maturity which nowadays is 12/13 could be was also later according to the length of peoples lives though this would have been verified in those days through physical signs.