Minimum age required: 40 years old. Also required is for your backstory to contain military experience and some traumatic events occuring.

You are a veteran of old conflicts, some of whom still haunt you. Your life has been long and hard-fought. Your experience and martial knowhow gives you an edge in combat. Where other, younger creatures, would not see the opportunities and trends, you do. When an attack roll misses, you can add your intelligence or wisdom modifier to that roll.

Images of your troubled past come to you in your sleep. When attempting a long rest, roll a d20 to see if you are able to rest comfortably and complete your long rest. If you fail, the long rest is replaced by a short rest instead. The DC for this roll is determined by the circumstances and should be decided by the dungeon master.