"I am the youngest of the family" A direct translation that works, but still got marked wrong. Come on Duo!!!!
Either it's fixed now (because that's what I put, and it was marked correct), or it was one of those things that someone at DL told me happens occasionally (theoretically not often), that someone is marked incorrect when they are correct.
That is exactly what I did, and it would be normal to say that, more so than "the youngest one". It was the only one marked wrong in my test out too so it hurt more!
Why not el menor? Or la menor but you can't assume anything in Duo from the speaker's gender.
Menor should work as well here, but it tends to have a connotation of "miniscule" or "less important", so "más joven" is often used instead.
Spanish: Yo soy El (masculine) más joven..
Yo soy LA ( female) más joven..
English I am the youngest girl/ boy in the family . I suppose
The youngest one - el o la más joven. ¿La camisa roja o la ( camisa) azul? The red shirt or the blue shirt.? ¿La de color rojo/ la roja o la de color azul / la azul? ¿Ésta o esa o aquella? - The red one or the blue one? This one or that one?
For this particular instance, you're selecting one item from a group. That needs de in Spanish.
English is often uncomfortable with using adjectives as nouns, so it often adds a dummy "one" to mitigate the awkwardness. "Which one do you like?" instead of "Which do you like?", or "The red one is nice" instead of "The red is nice."
But for superlative adjectives it's usually not as awkward, so "I am the youngest in the family" is just as good.