«فَهَذَا هُوَ الْمُخْتَلِطُ»
Such a narrator is described as mukhtaliṭ.
Ikhtilāṭ is a serious later impairment that disrupts an earlier state of sound transmission. It may follow cognitive decline, loss of vision where correct use of written material is affected, or the destruction or disappearance of reliable notebooks.
Old age alone does not establish ikhtilāṭ. Critics need evidence of a real change in transmission and, where possible, its timing. A narrator may have transmitted accurately from carefully preserved books for many years but become confused after those resources were lost.