I think this is the first time I'm inclined to disagree with you, Luis - if you are addressing a group of people, all of whom are young, then I feel either should be fine. If there is someone in the group who is not young, then I feel neither is appropriate. Either way, I think including "all" in either postion is quite unnatural in a sentence like this, and would only be used for emphasis - which might be where Todos would be used anyway...
Ustéd es joven. --- You are young.
Ustedes son jóvenes. --- You (all) are young.
Tú eres joven. --- You are young.
Ellos son jóvenes. --- They are young.
Ustedes son jóvenes. --- You are young.
PS: Note that joven gets an accent (una tilde) in plural form jóvenes, this is because all the proparoxytone words (palabras esdrújulas) are always accented in the stressed syllable.
Hola greenmachine: The stress on the singular is on the first syllable "JO-ven". When you make it plural, you must keep the same stress, If you do not put an accent mark on the plural, it would be "jo-VEN-es", but we do not want to pronounce it that way so we put an accent over the syllable we want stressed - the FIRST syllable just like the singular...........so we get "JO-venes" (jóvenes). The rules for stress are really quite easy: Three rules: 1. If a word ends in aeiou n or s, the stress goes on the second to the last syllable. ///////2. If a word ends in a consonant other than an "n" or an "s" the stress falls on the last syllable./////3. If the stress does not follow Rule 1 or 2, you must mark the stress with an accent mark.
This has to do with the rule for when to put accents. Accents are used to mark which is syllable in the word should be stressed. When the strongest syllable is the third to last, you always put an accent on the vowel (as is the case in jó-ve-nes). However, when the strongest syllable is the second to last (as in jo-ven), you only put an accent in the vowel when the word does not end in an "s", "n" or a vowel.
The rules for accents are somewhat complex, and nobody will fault you for making occasional accent mistakes (most native speakers do). For now, all you need to know is that the plural of joven is jóvenes.
This sentence also means "you are young people", right?
I was taught in Mexico that adjectives can stand alone as nouns when talking about people.
We do it in English, like "the accused" "the deceased" "the rich" etc, but it seems to be a lot more common (or at least more variety in application) in Spanish, or, Mexican Spanish at the least.