If a boy (teenager) learned he will not be punishesd until he is 20 and decides to go off the derech with the plan to do teshuvah at 20, would HaSh-m accept his teshuva?
The implication of the last mishna in Yoma is that the answer is no:
האומר, אחטא ואשוב, אחטא ואשוב, אין מספיקין בידו לעשות תשובה.
One who says, "I will sin, and then repent, I will sin, and then repent," will not receive an opportunity to repent.
This is codified by Maimonides is the Laws of Repentance 4:1:
אַרְבָעָה וְעֶשְׂרִים דְּבָרִים מְעַכְּבִין אֶת הַתְּשׁוּבָה. אַרְבָּעָה מֵהֶן עָוֹן גָּדוֹל וְהָעוֹשֶׂה אֶחָד מֵהֶן אֵין הַקָּדוֹשׁ בָּרוּךְ הוּא מַסְפִּיק בְּיָדוֹ לַעֲשׂוֹת תְּשׁוּבָה לְפִי גֹּדֶל חֶטְאוֹ. וְאֵלּוּ הֵן... ד) וְהָאוֹמֵר אֶחְטָא וְאָשׁוּב ...
Twenty-four things prevent teshuvah. Four of them are a great sin and anyone who does one of them, the Holy Blessed One does not enable that person to repent because of the seriousness of his sin, and they are: ... and (4) someone who says, “I will sin and I will repent,” ...
I would imagine the Mishna in Yoma presupposes that we are discussing an intelligent adult and therefore not an applicable halacha here...The whole premise of "the under 20 pass" is the lack of foresight and intelligence a teenager has, see this http://mariacapaz.pt/wp-content/uploads/2015/06/Pris%C3%A3o-foto-5.jpg Simply put the under 20 teen makes mistakes so the outcome of his sins and business dealings are suspended even the postponing of repentance would fall under "the under 20 pass" and not under האומר, אחטא ואשוב, אחטא ואשוב, אין מספיקין בידו לעשות תשובה.