yad ardham ayusas tasya
purvah parardho ’pakranto
hy aparo ’dya pravartate
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The one hundred years of Brahma’s life are divided into two parts, the first half and the second half. The first half of the duration of Brahma’s life is already over, and the second half is now current.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The duration of one hundred years in the life of Brahma has already been discussed in many places in this work, and it is described in Bhagavad-gita (8.17) also. Fifty years of the life of Brahma are already over, and fifty years are yet to be completed; then, for Brahma also, death is inevitable.