The army of the early empire was sharply and qualitatively different. Augustus defined a permanent professional army by setting the enlistment at 16 years (with an additional 4 for reserve obligations), and establishing a special military fund, the aerarium militare, imposing a 5% inheritance tax and 1% auction sales tax to pay for it.
The Roman army during Augustus' reign was significantly different from previous armies.  He established a permanent professional force by setting the enlistment age at 16 years, with an additional four years for reserve duty. To fund this new structure, he created the aerarium militare, a special military fund, and imposed taxes on inheritance and auction sales.