Wassersug et al. (2004) have a (interesting? painful?) survey of 134 men with an interest in castration. Here are some numbers:

Age

Under 25: 4%
25–35: 16%
35–45: 23%
45–55: 38%
Over 55: 17%
No answer provided: 2%

Education

Non-HS: 1%
HS graduate: 16%
2 yrs college: 22%
4 yrs college: 25%
Postcollege: 25%
Doctoral degree: 9%
No answer provided: 3%

Reasons respondents desire castration (by number of responses)

A feeling of calm, often called the eunuch calm: 42
A sense of control over one’s sexual urges and/or sexual appetite: 41
The excitement of the castration scene itself: 32
Cosmetic effect. Just like the look: 31
Feeling a deep desire to be submissive to partner: 26

Interesting that they have a much higher rate of advanced education than the general population.