I bought some equipment from a smithy and noticed that my heroes' age went up when I equipped them. I tried unequipping them to see if it would reverse the aging, but it didn't. Tried t he temples, but they were no help. I tried reading up the manual and clue book but I still couldn't find anything. Is there a spell to reverse the aging? Some shrine I have to pray at? Trying all I can to find a way to cure it, but so far I've been in no luck and I'm starting to get fearful that my heroes will die of old age.

In MM 7 and 8 there was a potion created by alchemisis which removed aging.
However I didn't play any MMs older than MM6, so I can't say that it will work there too.

What's the calendar date? Visiting a shop takes one day, so if you happened to visit on the last day of the year, your characters' age will increase.
There is a way to remove unnatural aging, but natural aging cannot be reversed. If your age is silver and the tooltip says "23 / 18" or something like that, you've been unnaturally aged.
Don't worry about dying of old age. Even if you waste a lot of time, it would probably be difficult to even reach 40 before you complete the game.

The aging isn't natural. For example, my Paladin's natural age is 21, but he's 46 years old. My other characters are aged 26, but their natural age is 21 as well. I want to know how I can revert this.

Alright, thanks. Time to crack open the dusty maps and start hiking, I guess.

***** SPOILER *****
***** SPOILER ***** (SERIOUSLY!!!)
It's a small island off the coast of the first island (Map area around B2 or B3), but the island is submerged until you raise it. You do this (as far as I remember - it's been close to 20 years) in the pyramid found a bit below the Good Castle on the first island.
Maybe someone else can be even more accurate but that's as much as I remember and I haven't gotten to that point in my own replay-through yet. Only one of my characters are slightly aged so it hasn't come up.
Also I would hazard a guess that what aged you were some fights - probably with Ghosts - before you actually entered the store. You often don't notice right away - I've tried adventuring for weeks before noticing ;)