Q. My dear young son asked me recently why we count Sefira for the days past after Pesach. Would it not make more sense to count the days before Shavuot we still have, so we can prepare for Kabalat Hatorah properly? At what age does one have to teach children to count the Omer?
A. Medresh Raba (Shemos 3: 12) mentions that the whole purpose of Yetzias Mitzraim (leaving Egypt) as mentioned in the above pasuk was to receive the Torah. The people then asked Moshe Rabenu when is that great moment that gives significance to the terrible solitude and spiritual impoverishment we are so long suffering now going to happen? And Moshe answered at the end of fifty days. So they began counting then and in that form of the greatest hope, desire and anticipation for that incredibly great future.
Sefer Hachinuch (Mitzva 306) explains that the Sefira represents the strong desire of our nation and preparation for Kabalat Hatorah. Similar to one expecting to receive the greatest prize and award desired, especially when one wants to run away or fix the most terrible past.
It also establishes the principle that the fixing of the past is essential for the planing to a great future time.
If the child understands the basics of the above, he should already count.
Rabbi A. Bartfeld as revised by, Horav Yaakov Hirschman, Horav Dovid Pam, Horav Aharon Miller, Horav Chanoch Ehrentreu and Horav Kalman Ochs Shlit'a