Thank you for your comment and for pointing that out. I checked the Bagnall paper again, and you are right.

Here's a quote: "Among 49 children 1 to 5 years of age, ...30 of 33 cases (91%) occurred during sleep...In contrast, among 173 persons 31 to 35 years of age,...10 of 31 cases (32%) occurred during sleep."

But I'm not aware of any further breakdown of this data, unfortunately.