In recent campaigns we usually abandoned EXP and were just 'granted' a level at certain points, usually after a 2-4 sessions, depending on the encounters, so it would need some adaptations in those type of campaigns, but that is up to the DM of course. Maybe add a small note to it, saying they are just 'guidelines' for 'regular' campaigns.
With great power, comes great responsability .Peasants are probably level 1 commoners with a basic stat array. If stats are randomly distributed, maybe 30% of them have 12 STR. A Deathwalker would need to personally kill upwards of 240 peasants to gain +1 to strength... If someone does that they will have an entire kingdom's military chasing them down just as fast as a dragon that was destroying villages would.
When Cheat Death becomes an option, an Evil Deathwalker could wipe out a village of 400 singlehandedly to reduce his age by a year. At that point, you're either running an evil campaign or he's the new BBEG.
*Brain fart* Now it suddenly makes a lot more sense . Still some will likely read it the way I did in the previous post. But again, that is up to the DM if he would 'allow' it.His physical age gets reduced by 1 year - his mental age is the same - so mentally he's still middle aged and doesn't get the mental bonuses again until he goes from middle aged to old.