Why were Yisrael counted only from the age of twenty?
Rashi: Because they were only conscripted and counted as men from the age of twenty. 1
Oznayim la'Torah: Because the Machtzis ha'Shekel came to atone for the sin of the Eigel, and it is only from the age of twenty that one is punisheable at the hand of Beis-Din shel Ma'alah.
See Sifsei Chachamim.
What is the significance of "Kol ha'Over al ha'Pekudim"?
Rashi (in Shmuel 2, 12:5): It refers to the annual half-Shekel donation.
To whom does "Kol ha'Over al ha'Pekudim" pertain?
Why here, does the Torah writes "Kol ha'Over al ha'Pekudim", and in Pikudei, in 38:26, "le'Chol ha'Over..."!
Rashi (in 14:28) and Riva (in 27:3): Sometimes the Torah adds a 'Lamed' that is not needed. 1
Moshav Zekenim (citing R. Yehudah ha'Chasid): "Kol" refers to givers, whereaas "le'Chol" hints at receivers. Everyone gave [items of silver] according to what he judged himself capable, from which Moshe made coins. Subsequently, he gave everyone a coin asa gift, and each person then donated his coin.
Rashi and Riva give examples of other such Pasukim, though this is not one of them.
Why does the Torah write "Kol ha'Over al ha'Pekudim", and not 'im ha'Pekudim' (Whoever passes into the place of those who were counted must give)?
Hadar Zekenim: We find that sometimes the Torah writes "Al" instead of 'Im'.
