What age are trolls considered Adults and sent to do their Ordeals?

Likely somewhere around the Human equivalent of 20-21 or maybe a bit later. The Dancestors are all 9 sweeps old, which is 19.5 Human Years, and while it could be a stylistic choice, they do not seem Adults, they are older than the Kids, but they are still not on the “Adult” sort of spectrum. So yeah, 20-21, maybe 23-24 at most? 10 Sweeps sounds round enough. 11 Sweeps could tie in with the Cherub Numerals, with the LE-Controlled Alternia.