Diffuse texture vs other techniques is a false dichotomy (there should be aging in both in most cases), but if it's realism you're after, the latter is FAR more important.

Why? Because doing all the aging in the texture means you're just faking wrinkles and facial shape. That will only look real under a very limited set of conditions as far as distance, posing and lighting. Once it becomes clear that the baked-in shadowing is wrong, or the angle demonstrates that the mesh actually is flat/smooth where the wrinkles and juts are painted in (and these are all things very easy to discern) - sorry, that doesn't look genuine at all. It may still look like a character intended to be old - but it won't look realistic in the slightest. (To me, it looks a bit silly, to be honest.) Wrinkles and other facial deformations associated with age must impact the mesh whether through morphs or displacement or both (although there are some things you can get away with by using a good bump).

In comparison, altering the diffuse texture can aid in the aging process in the following ways:

1. subtle changes in skin colour as one ages

2. frequent spot coloration of the skin, especially with the actual elderly (less so with the 40-50 range, but it can be there too)

3. very subtle wrinkle details that wouldn't require even bump

Etc.

Those diffuse texture details are just that - (1) diffuse - it's there for colour detail, not shaping; and (2) details that can put the finishing touches on realism. But without the mesh being affected, you're not in position to do :"finishing touches" - if, again, realism is your goal.

One final observation: If we're really talking realism I should add that some of the "older character" morphs I've seen for sale aren't realistic because they're too exaggerated. Most people don't turn into caricatures as they age; such morphs would have to be "backed off" in order to be usable in a realistic render.

IMO, FWIW, M2C and all that.

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