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.)
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.
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.