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SB 3.11.34

yad ardham āyuṣas tasya
 parārdham abhidhīyate
pūrvaḥ parārdho ’pakrānto
 hy aparo ’dya pravartate
Synonyms: 
yat — that which; ardham — half; āyuṣaḥ — of the duration of life; tasya — his; parārdham — a parārdha; abhidhīyate — is called; pūrvaḥ — the former; para-ardhaḥ — half of the duration of life; apakrāntaḥ — having passed; hi — certainly; aparaḥ — the latter; adya — in this millennium; pravartate — shall begin.
Translation: 
The one hundred years of Brahmā’s life are divided into two parts, the first half and the second half. The first half of the duration of Brahmā’s life is already over, and the second half is now current.
Purport: 

The duration of one hundred years in the life of Brahmā has already been discussed in many places in this work, and it is described in Bhagavad-gītā (8.17) also. Fifty years of the life of Brahmā are already over, and fifty years are yet to be completed; then, for Brahmā also, death is inevitable.