There are quests (one on Clouds, one on Darkside) to cure magical aging. Magical aging comes from various special events (some attacks; some scripted events) that age the victim without passing in-game time. Your problem appears to be that you passed in-game time, which cannot be cured. Your best option is to restore to a prior save game, from before you spent your characters' lives working. You could also try to finish the game as-is. Advanced age brings some statistic penalties, but depending on your equipment and other progress through the game, your party might be viable despite the penalties.

Problem with MM3-5 (unlike MM6-9) is that old age increases chances to die at any moment. I think that chance starts appearing at around 70ish, that's when characters have chance to die during resting. Eventually chance to die during rest becomes 100% and you can eventually die simply by traveling. I am not even speaking about huge stat penalty.

There is another way to partially fix stuff because we speak about MM 4-5. You can make 6 new characters. Use 3 old characters and 3 new characters to get some EXP from Warzone (you can go to traveling mirror and type Warzone - it is training arena where you can fight for EXP infinitely) or from Xeen's Castle (if you didn't destroy monster spawners yet). Do the same for other 3 characters. There is also a shrine in one of the easier dungeons of Darkside, which can increase all stats to 50 if they are below that, this way you can make up for permanent stat boosters which were used up by old party.

This way you can keep all progress.

Actually, I believe this is not the case. The only thing that can cause a character to die in their sleep is having one of their attributes at 0; if all attributes (including age modifiers) are 0 (or less, which will be displayed as 0), only then might the character die while sleeping (or just walking around).

From what I recall, at age 51, the age modifier is 20, and at age 101, it jumps to 50, so if you get a young character's stats (at least the ones that decrease with age) up to at least 51, the character will not die of old age. (Note that, near the end of MM5 there is a way to raise all stats less than 50 by 50, which can accomplish this form of immortality.) Age will lower Strength, Endurance, Accuracy, and Speed, so those are the attributes you need to worry about. Luck does not change, and Intelligence and Personality will actually increase. (I recommend giving effects that boost all stats to your Cleric so that she can eventually survive old age and can therefore use Divine Intervention freely, getting more Personality (and hence SP) in the process.)

You can't go there until the very end of the game.

Lowered attributes are actually a huge penalty in MM 3-5. No gimmicks will make up for that. Perhaps there are some exploits to circumvent that, but they don't count as viable approach. Perhaps I don't remember it well now, I thought it was in Temple of Bark.

In the case of the Cleric, however:
* Strength and Accuracy matter if you're attacking, but not if you are not.
* Speed might matter, but isn't too important if you have fighter-type characters with good speed.
* Endurance is the one stat that could be an issue; however, there is one dungeon in World of Xeen (Dragon Tower) where there's a total of 320 points of Endurance; surely you could spare some for your Cleric.
* Getting Personality up to 250 for the big spell point boost is not that easy in World of Xeen; hence, you might need the bonus from aging to get the stat up that high.
* Intelligence and Luck are irrelevant for this discussion; Luck doesn't change, and Intelligence (which is useless for a Cleric) doesn't decrease.

So, all you really need for this set-up is (before aging modifiers) 51+ Strength, Accuracy, and Speed, and as much Endurance as reasonable, for this to be viable for a Cleric.

Also, if you need more Endurance temporarily, there *is* the fountain of youth; but then again, if you are in the Southern Sphinx, you could probably just focus on getting your Cleric's AC up to around 250 or so and she should only rarely actually get hit. (You can ignore AC on the rest of your party here.) Anywhere else (late game), just use Divine Intervention and don't worry about aging.

I've loaded up a previous save that I had where my characters were 19 again.

I've done exploring out of one of the cities and covered about 20/30 square squares (with returning to the city to heal up) and my parties age became 22 in the time span of somewhat 30 minutes of play-time. (somewhat of an exaggeration)

This worries me to an extent due to game becoming limited in nature but...

Time is of the essence in this dungeon Sim and there are ways to go around this like making new party members as was suggested and mixing them up with the current party.

This doesn't necessarily take away from the game but make it more realistic in a sense because you've got even more attributes and stats to work and keep in consideration with.

Before, time wise; I was orienting on food. I had that food for a set amount of days and while the food didn't spoil, years passed by... so that didn't make sense and it threw me off logic wise whilst taking on odd jobs that I didn't notice years had passed by. Just when my party started dying (continously regardless of loading previous saves) at the age of 100 is when I realized I done messed up and that this game is deeper in a sense.

It makes you really work to progress. It makes you think and I love it.

I'm just torn up now because I was used to my party or play-character somewhat being immortal, so I didn't have to deal with this aspect but this game made me change my ways.

It just needs getting used to.

On the subject of money; I've noticed that banks give interest to your deposited gems and gold, so that's an aspect that sort of drives the game. This monetary simulation.

Game is so diverse, it tops a lot of modern action titles.

As usual: thanks for your tips and comments.

Certain actions can advance time much more rapidly. You should easily be able to finish the game while your party members are quite young, if you avoid unnecessarily advancing the clock.

The most likely candidates for rapid time passage are:
- Resting extensively. Every rest costs you 8 hours.
- Using town shops. Each use costs at least 1 day; training costs time based on how much you train.
- Wandering in the desert without a navigator. If none of your characters have navigation, every step costs 4 hours. If you have a navigator, or are not in the desert, every outdoor step costs 10 minutes. You should notice this one quickly because the sky will be cycling between day and night very often.

There are other ways to advance time rapidly, but none that I think you would have hit from your description.