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Consequently, the custom has arisen in Jewish circles to give donations and monetary gifts in multiples of 18 as an expression of blessing for long life as 18 is considered a blessed number because of it is the numerical value of “חי”.
In Judaism, in the Talmud; Pirkei Avot (5:25), Rabbi Yehudah Ben Teime gives the age of 18 as the appropriate age to get married (“Ben shmonah esra lechupah”, at eighteen years old to the Chupah (marriage canopy).