Barbarian peoples who do not till the soil have no territory of their own, and are, as we have said, governed more by the law of nations than by civil law. Therefore they are always armed. Thus Tacitus tells us that “the Germans never conducted any business, public or private, without being armed.”  They indicated their opinion  with a sign they made with their weapons.  As soon as they could bear them they were presented to the assembly ; a javelin was placed in their hands  : at that moment they left childhood behind ; they were part of the family, and became part of the republic. 
Childebert II was fifteen when his uncle Guntram declared him a major and able to govern by himself.  He said to him : “I have placed this javelin in thy hands as a sign that I have given thee my entire kingdom” ; and turning to face the assembly, continued : “You see that my son Childebert has become a man : obey him.” 
In the law of the Ripuarians we see this age of fifteen, the ability to bear arms, and majority all going together. “If a Ripuarian has died or been killed,” it is said there, “and has left a son, he may not prosecute or be prosecuted until he is fully fifteen, at which time he will answer for himself or choose a champion.”  His mind had to be sufficiently formed to defend himself in court, and the body also to defend itself in combat. Among the Burgundians,  who also had the practice of combat in judiciary actions, majority was again at fifteen.
Agathias tells us that the weapons of the Franks were light. They could therefore attain majority at fifteen. Subsequently the weapons became heavy, and were already quite so in Charlemagne’s time, as is apparent in our capitularies and romances : those who had fiefs,  and who therefore had to do military service, no longer attained majority before the age of twenty-one.