He who did not learn Torah in his youth and seeks to learn in his old age but cannot
The appropriate time for learning is in childhood when the mind develops and cultivates intellectual skills. In old age, it is too late to develop learning abilities that were not acquired in youth.
("He who is unable to learn Torah in his youth, and seeks to learn in his old age, but is unable" - Devarim Rava (Vilna), Parashah Nitzavim, Parashah 8)
Translation
Expression (Hebrew)
זֶה שֶׁאֵינוֹ לָמֵד תּוֹרָה בְּנַעֲרוּתוֹ וּמְבַקֵּשׁ לִלְמֹד בְּזִקְנוּתוֹ וְאֵינוֹ יָכוֹל
Definition (Hebrew)
הַזְּמַן הַמַּתְאִים לִלְמִידָה הוּא בַּיַּלְדוּת כְּשֶׁהַמּוֹחַ מִתְפַּתֵּחַ וּמְפַתֵּחַ כִּשּׁוּרִים שִׂכְלִיִּים. בְּגִיל הַזִּקְנָה מְאֻחָר לְפַתֵּחַ יְכוֹלוֹת לִמּוּדִיּוֹת שֶׁלֹּא נִרְכְּשׁוּ בַּיַּלְדוּת.
("זֶה שֶׁאֵינוֹ לָמַד תּוֹרָה בְּנַעֲרוּתוֹ וּמְבַקֵּשׁ לִלְמֹד בְּזִקְנוּתוֹ וְאֵינוֹ יָכוֹל"- דְּבָרִים רַבָּה (וִילְנָא) פָּרָשַׁת נִצְבִּים פָּרָשָׁה ח)