The early gestures of the spell were complex and precise; you twitched your wand once, twice, thrice, and four times with small tilts at exactly the right relative angles, you shifted your forefinger and thumb exactly the right distances...

The Ministry thought this meant it was futile to try and teach anyone the spell before their fifth year. There had been a few known cases of younger children learning it, and this had been dismissed as "genius".

It might not have been a very polite way of putting it, but Harry was beginning to see why Professor Quirrell had claimed that the Ministry Committee of Curriculum would have been of greater benefit to wizardkind if they had been used as landfill.

— Harry Potter and the Methods of Rationality by Eliezer Yudkowsky