Minimum average LDoN level is 20.
Maximum level difference of LDoN members is 10.
Because a group of 5 lvl24 and 1 lvl14 would be an average of {22 + 1/3}, the average group level may be over 20, even with a member below 15. Still, though I can't say for sure, I believe that's a also:
Minimum LDoN level is 15.