When referring to animate nouns, the comparative form старље is used for comparing only old people (or animals). In this sense it is like English 'aged, more aged'.
— Мћй дљдушка ћчень стђрый. My Grandpa is very old.
— А мћй ещё старље. Mine is older (more aged).
The other comparative form, стђрше, does not imply that either of the two people is old:
— Шџре 10 лљт. Shura is 10 years old.
— Єра стђрше егћ на двђ гћда. Yura is 2 years older than he is.