Really, calculating age short of forensic evidence is an art.
Two of my LGFs are currently 11 years old. They have a 4 month gap between them.
The older one does look older; but not 4 months older. Being generous on both sides, with the differences in height and body types, the older can pass as 14 or 15 and the younger as 9, maybe even 8. The visual 5 + years difference is certainly due to genetics; with some influence from habits I guess, but mostly genetic.
Of course the younger one will eventually look like a tween, teen and adult; but I assume that she will always look younger than the older one.