One would think that your heir(s) would also be levelling up whilst you are still alive.
You start the game aged 30, lets say you die at 60 (a hard life, arthritis and lots of micro-fractures on your skeleton from constant combat, lung problems from breathing crappy medieval smoke-filled air, stress from governing a fiefdom, gout from drinking too much as a fat old lord etc) and had your first heir at 35. that's still 25 years for you to train them, coach them, take them with you into wars across the continent and level them up to be something half decent, so that when you do finally kick the bucket you don't start off playing as a total pillock lol.