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.
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?
Since I can't report it using the mobile website, I'll make the comment here. Having a female voice and female pictured saying "yo soy el más joven" is confusing for beginners, unless "yo soy la más joven" is never correct, which is what I would have to assume if I didn't read other comments.
Terry, the word del is the contraction of de and the singular masculine article el. Any oher article won't contract. Since familia is a feminine noun, you cannot use del here, only "de la".
The same is true for al, which is a contraction of a with the masculine article el.