Correct, although Wagner has Isolde referred to as the Irish/Ireland's child (Irisch/Irlands Kind) and as a Maid at various points in the opera; not conclusive, but suggestive of a young woman. In Gottfried von Strassburg's romance (Arthur Hatto's translation), she is often referred to as young. As a princess of marriageable age in the 12th Century, Gottfried's audience would probably have envisioned her as a teenager in any case, even if her age isn't specified. Likewise, Gottfried describes Tristan as a "young lord" and an "agreeable young man".