There was a punctuation error. Which it is crucial for understanding the semantics in the context that wants to be expressed:
1) Yo. Un poco cansado. ¿Y usted?
    Yo. Muy bien. 
    (One subject does not want to talk. The other answering subject (whether old or young) seems all right. 
2) Yo... Un poco cansado. ¿Y usted?
     Yo. Muy bien.
    (One subject is distracted and does not want to talk.  The other answering subject (whether old or young) seems all right.
3)  ¡Yo! Un poco cansado. ¿Y usted? 
     Yo. Muy bien. 
     (One subject is surprised that the other person is asking but does not want to talk.  The other answering subject (whether old or young) seems all right.
4)  ¡Yo...! Un poco cansado. ¿Y usted?
     Yo... Muy... bien.
     (First subject is distracted and surprised of the second subject's question but does not want to talk. The second subject answers with a surprise too and depending of the expressions/ relations it may be inferred a sarcasm of the second subject.) To summarize, you may write with or without 'yo' and replace it for 'estoy'. However, punctuation is a key factor to express even the mood of a subject. There are many other punctuation ways that I did not mention.
NOTE: You made a mistake with the sentence structure and I quote: 
"Yo un poco cansado. ¿Y ustedes?
Yo muy bien"
INSTEAD,
—Yo un poco cansado. ¿Y USTEDES?
—NOSOTROS muy bien
OR,
—Yo un poco cansado. ¿Y USTED?
—YO muy bien
OR
—Yo un poco cansado. ¿Y USTEDES? [1ST SUBJECT]
—YO muy bien [2ND SUBJECT]
—YO muy bien [3RD SUBJECT]