A pair (or more) with a highly-visible apparent difference between their ages, based on visual cues such as physical maturity, size, etc.
If the difference in ages is not readily-apparent from the image itself (by cues such as physical maturity, size, etc), do not use this tag. Similarly, the tag may be used for any picture where there appears to be a wide gap in age, as long as that is the logical conclusion from the image itself.
As a rough guideline, add seven years to half of Lover's apparent age. If this value is greater than Beloved's apparent age, then age_difference may apply, although any very visible difference in ages is appropriate for the tag's use.
source: danbooru wiki