If I recall correctly, the sources say the idol was created grown up and fully functioning.
Even if it was still in infant stages, Bava Kamma 65b:
אמר רבא שור בן יומו קרוי שור
(DonielF) according to Rashi (Shemos 32:4) they took Michah's golden plaque that said עלי שור on it and threw it into the fire, so a שור must come out.
I wonder why the idol is called everywhere "עגל" - calf and not a grown-up "שור" - bull?