Jamieson, Fausset and Brown on 1 Chronicles 27:23

23. But David took not the number of them from twenty years old and under--The census which David ordered did not extend to all the Israelites; for to contemplate such an enumeration would have been to attempt an impossibility (Ge 28:14), and besides would have been a daring offense to God. The limitation to a certain age was what had probably quieted David's conscience as to the lawfulness of the measure, while its expediency was strongly pressed upon his mind by the army arrangements he had in view.