The younger the better. If you think its difficult when she’s young, it gets harder and harder, for many reasons. Sometimes girls do excellent Shidduchim when they’re older, but thats not the norm.
Also imparting to our boys and girls that they wont be young forever. Often young people feel that theres no urgency to get married when something good comes along, because good offers will come along later too. Not always.
oomis, in the crowds where there is mixed socialization, there are just as many, if not more, older singles.