You can never have -18 as you only get -1 in each age.
E.g. in the third age, you get +5 as the winner and -1 as the loser of a conflict.
Therefore if you win the military check in all three ages, each of your neighbor has only -3 from you plus a maximum of -3 from the other players, giving you 12 points maximum.
Doh, we were doing age I is negative one, age II is negative 3, age III is negative 5.
Thank you. She still would have crushed me.