all things will wear with time, and time alone can weaken about any thing, the issue becomes more of a GAMBLE as time passes
at what point to do you feel is old enough to replace, as the human eye cannot see the damages a shaft can have, all them sudden stops,m friction rubs in and out of targets, does play into wear and tear, and can structurally damage things
as the shafts DO flex on launch and stop
slight damages can lead to MAJOR failures if things line up right!
I personally retire my arrows/bolts after killing a deer with them, I know MANY DON"T, and re use over and over again!
but to each there own, but damage does occur, and its many times the damage you DON"T see that get you!
as for repairing a shaft
I do NOT Know of how anyone could do so, its made once and that's it
you can re feather things, and change/replace inserts and or nocks
but that's NOT repairing a shaft, that's making the arrow able to be shot again and or fly better!
as for boradheads
they need to be re sharpened as they get dull, how fast they get dull depends on what they are shot into
and they do / can be repaired some what , as in one's that have replaceable parts, but NOT all do
and filling off metal in the process of sharpening things can also effect balance on them
so HOW Much can you remove before things get effected??
that's a unknown impossible question, as a lot comes down to HOW the head/blades are damaged and how much metal has to be removed to correct
if the MAIN body of broad head gets bent, things can get damaged past being able to save, making them useless
I personally think things just need to be examined as they happen and make decisions based on the damages done to things
pure aluminum shafts are more prone to damage too, than carbon/carbon wrapped aluminum shafts
but they all can be damaged to being in an unsafe state
best advice, MIGHT be to look up your arrow makers info on THERE product, and follow there safety recommendations, over advice given freely on a tread!
they spent a LOT of money testing things, and have a liability worry to protect, so there advice will be more accurate than the average hunters shooters, and again things can vary, from brand to brand,a s there NOT all made of the same materials or process!
I put safety always before trying to get more use out of anything!
some folks again will take things closer to the edge than me,
I don;t drive on low tread tires, I don;t wait for a blow out to tell me to replace them, I replace when I feel its time
as an example here
it can be personal, and or you can USE the OEM"S recommendation to make YOUR decision!