King Solomon wrote in Proverbs 32:6: "Train up a child in the way he should go; even when he is old he will not depart from it.".
What does "his way" mean? Apparently, it should say according to the path of the Torah, or ethics, etc.
Every child has natural qualities, some positive and some negative. The work of education (which differs from that of teaching) consists of guiding the child to develop good midot—emotional qualities—and to eliminate or modify their negative qualities.
The "according to his way" style of education involves accustoming the child to practicing good attitudes and acquiring new positive qualities that he did not possess naturally, so that these become "his way," since the habit itself becomes second nature. In this way, "even when he grows old, he will not depart from it," from good attitudes.
Unfortunately, the same applies in reverse: if educators don't address a child's negative attitudes as early as possible, these will become ingrained, and the older they grow, the harder it will be to change them, since we have the promise that "even when they grow old, they will not depart from it." (It's worth noting that nothing prevents Teshuvah—repentance and personal improvement—but this isn't a license to put off until tomorrow what one can do today, since one might not have time tomorrow...).