In Bereishis 21:8 the pasuk writes:
וַיִּגְדַּ֥ל הַיֶּ֖לֶד וַיִּגָּמַ֑ל וַיַּ֤עַשׂ אַבְרָהָם֙ מִשְׁתֶּ֣ה גָד֔וֹל בְּי֖וֹם הִגָּמֵ֥ל אֶת־יִצְחָֽק׃
The child grew up and was weaned, and Abraham held a great feast on the day that Isaac was weaned.
Rashi there clarifies that this was when Yitzchak was age two years old (24 months).
ויגמל. לְסוֹף כ"ד חֹדֶשׁ (גיטין ע"ה)
ויגמל AND HE WAS WEANED — at the expiration of twenty- four months (Gittin 75b).
When the Torah introduces the Akeida it writes in Bereishis 22:1:
וַיְהִ֗י אַחַר֙ הַדְּבָרִ֣ים הָאֵ֔לֶּה וְהָ֣אֱלֹהִ֔ים נִסָּ֖ה אֶת־אַבְרָהָ֑ם וַיֹּ֣אמֶר אֵלָ֔יו אַבְרָהָ֖ם וַיֹּ֥אמֶר הִנֵּֽנִי׃
Some time afterward, G-d put Abraham to the test. He said to him, “Abraham,” and he answered, “Here I am.”
Rashi over there mentions that the Satan took the opportunity to denounce Avraham by referring to his previous feasts and how Avraham failed to bring a korban (sacrifice) to Hashem:
אחר הדברים האלה. יֵשׁ מֵרַבּוֹתֵינוּ אוֹמְרִים (סנהדרין פ"ט) אַחַר דְּבָרָיו שֶׁל שָׂטָן, שֶׁהָיָה מְקַטְרֵג וְאוֹמֵר מִכָּל סְעוּדָה שֶׁעָשָׂה אַבְרָהָם לֹא הִקְרִיב לְפָנֶיךָ פַּר אֶחָד אוֹ אַיִל אֶחָד; אָמַר לוֹ כְּלוּם עָשָׂה אֶלָּא בִּשְׁבִיל בְּנוֹ, אִלּוּ הָיִיתִי אוֹמֵר לוֹ זְבַח אוֹתוֹ לְפָנַי לֹא הָיָה מְעַכֵּב
אחר הדברים האלה AFTER THESE THINGS [or, WORDS] —Some of our Rabbis say (Sanhedrin 89b) that it means after the words of Satan who denounced Abraham saying, “Of all the banquets which Abraham prepared not a single bullock nor a single ram did he bring as a sacrifice to You ’. G-d replied to him, “Does he do anything at all except for his son’s sake? Yet if I were to bid him, “Sacrifice him to Me’’, he would not refuse’’.
The Gemara quoted by Rashi points out that this is a reference to the feast that the Avraham made when Yitzchak was two.
If we say that Yitzchak was aged 37 at the akeidah (Refer to Seder Olam Rabbah who makes the calculation as well as Rashi on Bereishis 25:20) that would mean that the Satan waited 35 years to make this claim!
Is there any reason given by the mefarshim as to why there was such a delay? Was it simply because Avraham had to past the other nine tests and once he did that, he could move on the final and hardest one?