I received a comment on one of my recent YouTube videos asking if "hard cast bullets lose their tensile strength over time?"  I initially thought "of course not, that's crazy," but I have to admit I'm no metallurgist, and I'm not going to make up an answer that I'm not 100% sure about and claim it's fact.

So, what say you?  Will a 21bhn slug still be 21bhn after ten years on the shelf?

Nope .

Lead can change hardness over time depending on the alloy. But from what I think I've seen it gets harder over time not softer.

Click on this link...  While the graph at the bottom is a heat treating experiment it shows changes in lead hardness over time.  Heat treating merely speeds up the rate of change.

I too read somewhere that it gradually gets harder, takes a while.

Thank you very much for the info!

Depends on the lead alloy.
A typical alloy using wheel weights or other source of antimony for hardness that also has a very little bit of arsenic in it (like 0.01% total arsenic in the lead alloy - and almost all antimony and many sources of lead will have some arsenic in it) that gets dropped from the mold into water (essentially heat-treating the bullet) will take a couple days to reach full hardness, then after a few years will start to soften and after a decade or more will reach a stable hardness that won't change any more.
If dropped to rag pile and allowed to slowly air-cool the bullets won't harden, but then won't soften either.
Pure lead-tin alloys don't exhibit this behavior, you need the antimony & arsenic for heat-treating to be possible.
It's been discovered that it's the antimony & arsenic together that allows lead alloys to be heat-treated for extra hardness, but the tempering temperature is very low and the bullets will very slowly anneal over time at normal room temperatures. Several good posts about this over at Cast Bullets forums.

Anecdotal evidence follows: I recently discovered a stash of 122’gr hardcast lead bullets. They’ve been around my place for over 20 years. In the past they exhibited very low leading at reasonable 9mm velocities. When I loaded them recently to similar past velocities they’ve been leading like they were almost swaged lead.  (Not quite that soft per the fingernail test but soft)

If they're water quenched to get the hardness then yeah.  They reach peak at something like days and slowly go back down over a couple years.

So I need to make sure I shoot my Underwood 158gr 38's every so often and replace them?