In The Syotaroh (1996) Manda Ringo surveyed almost a hundred shota fans (63 women, 18 men, 1 okama) about their favourite age for a shota character. The respondents were mainly in their late teens and early twenties.
As you can see in the graph, there was a clear preference for age 12, followed by 13, with a certain span between 10 and 14, whereas popularity drops at 15 and 16.
The ages were calculated from the age span chosen by the respondent: If someone checked 13–15, then age 14 was used in the graph. If the user checked 12–13, age 12 was used if they also chose elementary schooler, but age 13 if they chose middle schooler. The preferred ages ranged from 3 to 30, with a median of 16.
This corresponds well with the results of my own interviews, which are partly published in Shota Ripōto 1. The age spans 10–14 (仁さん/Jin-san), 9–15 (大地くん/Daichi-kun), and 11–13 (タケルさん/Takeru-san) all center on 12, whereas 勇太さん/Yuuta-san (10–15) goes halfway up to 13 and an anonymous 27-year-old man centers on 13 (10–16). シンジさん/Shinji-san centers on 14 (12–16).
What’s your favourite age of shota boys?
I have worked out a span between a minimum of 6 and a maximum of 16. But the ideal range is between 9 and 13, with the peak in 11 or more properly in the passage from 11 to 12.