Question
How old was Zachariah when his wife gave birth to John the Baptist?
Answer
The Gospel does not mention figure and only informs that Zachariah was in advanced age at the time of Gabriel visit.
And they had no child, because Elizabeth was barren, and they both were now well advanced in years. (Lukas 1: 7)
And Zachariah said unto the angel, How shall I know this? for I am an old man, and my wife well advanced in years. (Lukas 1: 18)
He was as old as his wife (Luke 1: 7), both of them are in advanced age.
How old were they?
Advanced age, old age, or elderly age begin at 60 years old.1

Meanwhile the oldest woman to give birth is 70 years old.
Hence, Zachariah and his wife’s ages are possible to quantified, i.e, ranging from 60 to 70 years old.
Bibliography
1Forman DE, Berman AD, McCabe CH, Baim DS, Wei JY, “PTCA in the Elderly: the ‘young-old’ versus the ‘old-old’” at http://www.ncbi.nlm.nih.gov/pubmed/1727842 ; access: January 20, 2015 3:51 UTC/ GMT
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