On the tab "My Preferences", accessible from "My Account", you can specify the age range and the gender of people that you want to cross paths with.
Preferences are completely reciprocal: you will never be visible to a person that doesn't match your criteria.
Once your preferences are set, please wait until you cross paths with new profiles that match your criteria (please note that you cannot go back in time: the new preferences will apply to your future crossings and the profiles you’ve crossed paths with before will remain in your Timeline).