I share this 13 years after this post
The Torah tells us that Avraham made a Big Party (משתה גדול) – ביום הגמל את יצחק. The timeframe is unclear, leaving at least 3 possibilities as to what was being celebrated about Yitzchak: 1. Being weaned from the breast, 2. His Circumcision – this is the Midrashic homily as a play on the word הגמל that ה (5) + ג (3) is the day when he was מל (circumcised), (Pesikta suggests Avraham actually made two parties reflecting those first two possibilities,) (see also here) 3. When he was weaned from the Yetzer Hara at the age of 13. (Midrash Aggadah)
Of course, if the latter is true, this would be the second time Avraham had an event at a time his son was 13, as Yishmael was circumcised at that age in Chapter 17.
Both of these 13-year-old anecdotes indicate that 13 was a special age for a young man to reach, as it accompanied a heightened level of spiritual maturity. For Yishmael, the ability to accept being circumcised could be viewed more as a choice than something his father forced upon him, and for Yitzchak, being gifted a Yetzer HaTov to counter his Yetzer HaRa demonstrated the greater heights he could achieve with his newfound reality.
The Talmud in Kiddushin reminds us that גדול המצווה ועושה יותר ממי שאינו מצווה ועושה, that someone who is commanded and does has greater merits than someone who does anything strictly on a voluntary basis. Tosafos explain that one who is commanded worries more about doing the wrong thing and possibly violating, whereas the volunteer doesn’t care if something is done imperfectly. Ritva mentions two more reasons: The commanded is fulfilling the will of the king, while the volunteer wasn’t commanded to do anything. The commanded has the Satan who is trying to trip him up and do the wrong thing. Since לפום צערא אגרא, his effort is more rewarded.
Our opening Midrash indicates a weaning from the Yetzer Hara – perhaps it means that Avraham saw Yitzchak had accepted the responsibilities that came with his status of being 13 and had reason to celebrate!
Mazal tov!