The case can be made that the nuance of mannen is better conveyed in English by thousand than ten thousand, the former being commonly used to indicate long periods of time, as in not in a thousand years and face that launched a thousand ships. Thousand-year ko also appears in the literature, including the Go Player's Almanac.
However, ten thousand year ko remains the standard English for this term.
Suggestions for alternative English terms include:
Their merits aside, none has ever been used outside this page.
Retired English teachers with too much time on their hands have insisted that correct English punctuation style would be to use hyphens, as in "ten-thousand-year ko". Others differ, noting that the term is not hyphenated in the literature.
The Japanese is sometimes transliterated as mannenko, but it's recommended this term be hyphenated in English (mannen-ko) to highlight the fact that it's a type of ko.
This numeric form of the term is encountered sometimes, but for consistency it's better to stick with ten thousand year ko.