Board Thread:General Discussion/@comment-3574574-20180425194149/@comment-91424-20180425200449

Robin being eighteen in the flashback is only possible if Robin was magically aged up or has been spending time in a realm where time moves more quickly than the Land Without Magic (we do know that such realms do exist, such as the New Enchanted Forest and the Edge of Realms). Regina and Zelena do in fact indicate that something is off with the timelines, in "The Eighth Witch", where Robin is twenty-five:

Regina: "And Robin? You're, what, twenty-three now?

Robin: "Twenty-five." Regina: "She sure has grown up fast." Zelena: "Not in our realm." Regina: "Right. Well, I'm not here to discuss timelines."