Invest in Adult diapers
Yeah .-. About like maybe 13-17% of the population is into the stuff we are but you won't make much investing in that.
I mean incontinence briefs and underwear has too many regulations, they aren't interested in either thicker or crinklier. Basically any proposal that doesn't decrease cost, pad thickness, or doesn't make the product quieter is going to be ignored (or laughed at( by the major players in the US. The aging incontinent population being the big group you need to target is not interested in diapers that look like diapers, so if you found a company interested in the advancement of making something more like baby diapers, you lose most of this group right away (like 90%). I'm a bit curious about what numbers you looked at that made you state this, do you know of some pad advancement that could possibly be worth significant money as a patent? something to make it look more like underwear and less likely to share odors... these are possibly things worth investing in.
I have wondered if it was worth buying some diapers and storing them for 30+ years. Some of the oldest pampers sell sporadically for over $100 on eBay. Probably better off buying gold or bonds?
From a stock market perspective I'd honestly say that's a terrible investment. Typically to see a decent return on something you either want to buy into something that's growing or is at least volatile, or short something that's overvalued right before it fails. Diapers are likely neither. There's no reason to expect a sudden boom or sink in demand.
From a business perspective I'd echo what everyone else has said. You're not going to be able to compete with the big guys for the main markets (babies, seniors, hospitals, etc), and the niche "enthusiast" market is tiny, start up costs are high, and there's already plenty of companies in the market. There's no reason to think it's a sure path to riches.. you'd be lucky to make it to survival.
Gold if you time it right can be profitable. Bonds are fine as a hedge against volatility (I keep a small percentage of my portfolio in bonds and plan to grow it as I get closer to retirement) but you're (probably) not going to get any real return off them.
If you'd invested in the old Goodnites you'd be getting rich on eBay. Those vintage packs sell for easily 10x the retail price.
The cost of storage for a bulky item is non-trivial over time. I don't expect to be making much money off my stash.
I mean if you buy them then sell them 10 years later for 10x profit, sure. If you buy them and sell them 30 years later, you'd probably have made out better investing the money traditionally and letting it compound over time, especially when you factor in inflation, storage costs, selling costs, etc. Not to mention you have to really get lucky in the "buy something and hope it becomes a collectable" game... most of the time you end up with something that gets 50c at a yard sale _if_ the right person walks by.