Split infinitives occur when additional words are included between to and the verb in an infinitive. Many readers find a single adverb splitting the infinitive to be acceptable, but this practice should be avoided in formal writing.
When he was 11 years old, Itachi was also under orders from his father to enroll in the Anbu half a year later. Itachi was praised highly by Fugaku, who spent most of his time grooming Itachi to inherit the title of leader of the Uchiha, and hence inadvertently neglected his younger son.
In Naruto, the first half of the anime series that includes Naruto and Naruto Shippuden, Itachi Uchiha was 18. In Naruto Shippuden, he was 20 to 21. Official sources back this up. Itachi Uchiha.