This is taken as an instruction for Jewish parents to weigh their child's opinion in the balance during an arranged marriage.Regardless of whether proper procedure is followed, this is not the end of the decision - it is believed by Jews that the final say belongs to God, who may have different plans (compare with the match of Jacob and Leah).
Also the age when shidduchim start may vary by community.
In frum circles, especially among Hassidim, eighteen is the age when shidduchim start and shadchanim take notice.
Even so, Isaac gained his own impression of her before agreeing to marry her (Rashi, commentary to Genesis ).
However, when Eliezer proposes to take Rebekah back to Isaac in Canaan, he is told by Rebekah's family: "Let us ask the maiden" (i.e., Rebekah).
The Medieval Rabbi Nissim of Gerona (commonly called Ran) traces it back to the Aramaic word for "calm" (cf.