This should have a comma.

FROM:
His wife was old but he loved her.
TO:
His wife was old, but he loved her.

To 123xyz (and others): Sadly, in the U.S., youth is king and old age is not so much even a queen anymore. In this day and age of high divorce rates, I have often heard tales of a man leaving his wife for a younger woman. Rarely have I heard of a man leaving his wife for an older one. Though my comments may sound defensive, they are not directed at you or your comments. However, your comment did compel me to add context to the world from which this sentence was constructed. My hope in contributing that sentence was that it would appeal to the sentimental side of human nature that I would like to think we all still have.

To CK (and others): I am familiar with the rule about adding a comma before a conjunction in a compound sentence, but, at least in American English, the rule is optional when the independent clauses of the compound sentence are short. So that you don't have to take my word for it, I have included an excerpt from a book titled, "The Best Punctuation Book, Period: A Comprehensive Guide for Every Writer, Editor, Student, and Businessperson."

"When independent clauses joined by a conjunction are short and clear, the writer has the option of not using a comma."

Examples given are as follows:

"Jane likes pizza and she also likes pasta."
"You could stay or you could go."
"I walked there but I ran home."
"Vegetables are packed with vitamins and that's important."
"Pack your things and go."

I think I initially considered adding a comma to this sentence but from a visual/aesthetic point of view I felt it offset the symmetry and interfered with the poetic quality I think the sentence has without it. I suppose that is my own personal point of view, but as far as grammar is concerned, it appears that this sentence is in accordance with accepted rules of American English.