More than those, but genetic and environmental variation in their "cycles" (if you will) as well as the stochastic events involved in turning them over seems to produce extremely wide variation in their eventual readings.

I work on senescence in adult stem cells, for whatever that's worth. We have found 72-year-olds whose viable fractions were greater than those of 26-year-olds, but as far as we can tell (with sample sizes in the dozens instead of dozens of thousands, this isn't going to be much), there aren't a ton of good predictors save chemo & radiation.

YMMV