'Thirty' in the Bible

Also he made the cast metal sea, ten cubits from brim to brim, circular in form, and its height was five cubits and its circumference thirty cubits.

Now when Jehoiada reached a ripe old age he died; he was one hundred and thirty years old at his death.

without regard to their genealogical enrollment, to the males from thirty years old and upward—everyone who entered the house of the Lord for his daily obligations—for their work in their duties according to their divisions;