It is assumed that NPCs don't gain experience from doing adventuring stuff, but from living their lives in the normal fashion. Characters who only have NPC levels will be levelled roughly according to their age.
Commoners and Experts can be roughly expected to follow this pattern:
Characters with a lot of responsibility, or who have experienced an exceptional life, may progress quicker. Others may progress slower.
An Aristocrat will gain levels along with responsibility, which comes with rank, family size and wealth. This will roughly mirror the age based increased of Commoners, but be modified upwards for more powerful families.