This is my feeling, too. When dealing with somebody several centuries old who comes across as having a childlike sense of play and joy to mortal eyes, and the body of a svelte 25-year-old... that is part of the strangeness of Elves, not real childishness.
Somebody who is much younger, like Idril or Celebrimbor or Orodreth, is who I would picture as being actually innocent, or very naive, at this time. Although the Annals of Aman are problematic when it comes to Noldorin princes' birth dates, in the Annals Luthien is older than Finarfin. She is at least of his generation.