There was a punctuation error. Which it is crucial for understanding the semantics in the context that wants to be expressed:

1. One subject does not want to talk. The other answering subject (whether old or young) seems all right. 

- Yo. Un poco cansado. ¿Y usted?
- Yo. Muy bien. 


2. One subject is distracted and does not want to talk.  The other answering subject (whether old or young) seems all right.

- Yo... Un poco cansado. ¿Y usted?
- Yo. Muy bien.

3. 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.

- ¡Yo! Un poco cansado. ¿Y usted? 
- Yo. Muy bien. 


4. 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.

- ¡Yo...! Un poco cansado. ¿Y usted?
- Yo... Muy... bien.

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]