@whateversclevergreer: You wont see a tiara on any of the british princesses unless they are getting married or marry an aristo whose family has one. You wont find PE or PB wearing one unless either is the case.
That is likely the case for Eugenie and Beatrice because they’re not working royals and have no reason to wear a tiara but it’s untrue to say you won’t see a British princess in a tiara until they marry. Princess Margaret married in 1960. Here’s her in a tiara in 1951:
Anne married in 1973. Here’s her in a tiara in 1969:
Princess Alexandra married in 1963. Here’s her in a tiara in 1960: