There is no explicit proof, for example, anywhere in the play where Joe or Kate call Chris their older son, but he is older than Larry.  His age, 32, as compared to the younger Anne Deever at 26, is more consistent with George Deever, Anne's older brother who is 31.  Anne and Larry would have been around the same age, so Chris is the older brother. 

"Ann Deever, the attractive, twenty-six- year-old daughter of Joe’s former partner and neighbor, Steve Deever, whom she has not seen since his imprisonment. She was once Larry’s girlfriend but now is in love with Chris, who invited her back to her old neighborhood so he could propose to her."

Symbolically there is evidence to suggest that Larry was the younger son, the favorite of his mother, the baby of the family.  Chris is forced to be more practical, he must work for his father even though he is characterized as an idealist in the play, he has less freedom than Larry, who committed suicide, an act of ultimate freedom, even though it has negative connotations, that which Chris would never consider, as the more responsible and mature son. 

"Chris Keller, Joe’s thirty-two- year-old, sensitive, and intellectual son. He works for his father’s company, which someday will be his. A World War II veteran whose combat experience has left him with a strong sense of responsibility for others, he is an idealist, though rather naïve."

As the older, more responsible and grounded son, Chris feels like he is always sacrificing what he wants to please other people, younger sons usually don't have to do this, traditionally the younger siblings in the family get away with a lot more than the oldest.

Chris complains in the beginning of the play that he has always had to be more responsible.  He tells us that he has been a good son, a sucker too long, this suggests that he has followed whatever his parents wanted him to do.    

"I don't know why it is, but everytime I reach out for something I want, I have to pull back because other people will suffer. My whole bloody life, time after time, after time." (Miller)