Annius was made caesar on 12 October 166 AD, alongside his brother Commodus, designating them co-heirs of the Roman Empire.

[3][4] This was the first time such an explicit declaration of heirship had been made at such a young age,[5] and showed a marked shift from the traditional cursus honorum, in which a presumed heir would be gradually raised through offices of increasing importance, in order to learn the skills of all positions, to a new system of imperial succession, wherein dynastic hereditary descent was the path to the throne, with heirs being instructed in how to be an emperor.

[6] Annius died on 10 September 169 AD, at seven years of age, due to complications in removing a tumor from under his ear.

His lack of mourning was likely influenced by his Stoic philosophy, which taught of the dangers of emotion, and Roman cultural norms which treated hysterical grief at the loss of loved ones as unmanly and unnatural.

[10] Aurelius also had his name inserted into the Carmen Saliare, the ritual songs sung by the Salii, a group of 12 priests.