BACKGROUND:
Measures based on DNA methylation, epigenetic clocks, have recently gained attraction as predictors of mortality and age-related pathologies. However, the origins of variation in these measures are not well understood.

METHODS:
In a pooled sample of 104 Swedish and Danish twin pairs, we estimated, at the mean age of 70 (baseline) and 79 years (follow-up), the genetic and environmental influences on the Horvath and Levine clocks.

FINDINGS:
A model incorporating additive genetic (A) and person-specific environmental (E) influences best explained the variation in both clocks. Heritability was estimated at 55% at baseline and at 51% at follow-up for the Horvath clock and 34% at baseline and 41% at follow-up for the Levine clock. For the Horvath clock, new sources of A influences emerged at follow-up, whereas for the Levine clock, the same A influences accounted for the genetic variance at both measurement occasions. The cross-time phenotypic correlations, 0·52 for the Horvath clock and 0·36 for the Levine clock, were mediated primarily by genetic factors, whereas the person-specific environmental factors were completely different at the two measurement occasions.

INTERPRETATION:
For both clocks, new sources of person-specific environmental influences emerge with age. The epigenetic clocks might thus be responsive to new environmental stimuli even at old age.