At that age, it's probably something mechanical in the <ahem> mechanical switch.
But rotate or spin the batteries in the "cartridge" to re-establish good contact between the six (6, count 'em) contact surfaces in the battery pack.
Everybody laughs at me for suggesting it, but the aluminum-to-aluminum contact in the body to cap mating surfaces sometimes needs a little twist to break through any oxide layers between them. Don't forget that aluminum oxidizes very very very readily, and guess what is so good an insulator that they use it for insulators?
Aluminum oxide, that's what. AKA "alumina."
I don't know how many times a simple twist and twist-back of the endcap has restored the brightness (i.e., the contact) in those little freebie lights from Harbor Freight.*
Terry holds up hands to quell the laughter.
But yeah, at this late date, it's probably the switch.
Terry.... who never completely trusted aluminum connections anyway, 230RN
*I regularly pick up a couple of these almost every time I'm in Harbor Freight. I've got them everywhere you look in the house. Kind of a flashlight junkie for junkie flashlights.