Why did Moshe start to count from only the twenty years and above in Bamidbar 1?

Question

When counting the number of the Israelites in Bamidbar 1 Moshe only counts those who are twenty and above which somehow seems to be the age of responsibility. After having refused to enter Canaan all those who were counted died in the wilderness.

Numbers 1:3 NIV

You and Aaron are to count according to their divisions all the men in Israel who are twenty years old or more and able to serve in the army

But all Jewish men become responsible for their actions at the age of thirteen. But when Moshe counted the Israelites he only counted those who were twenty years old and above.

Could the age of responsibility have changed from the time of Moshe to the present day?

Answer 1

Those who were regarded as able to actively serve in the public domain were being counted. Those over sixty were in effect retired from the army. Thus when the punishment of the 40 years was decreed, those who would have served in the army (20 to 60) at that time were being punished. Those below the age of 20 would not have been eligible to be drafted.

Rav Hirsch says

צבא is altogether to be understood as every united body under higher orders for public service or for such services to be rendered by such a body. And here too one need not necessarily think of military service. יצא צבא ישראל would rather designate everyone who is in duty bound to step out of his simple private life into the public service of the community whenever it is demanded of him. Accordingly, everyone on whom the community can reckon to carry out its general concerns.

Thus we see that from the age of 13 (בר מצוה) to the age of 20, a person is expected to study and become mature enough to take his place in society. From the age of 20 to 60, he is to be capable of fulfilling his role in society (such as being eligible for conscription). At the age of 60, he is eligible to retire and take up a higher role in society and not be required for the domain of the army.