Snakes shed every 3-6 weeks normally.  Predicting age based on shedding is not accurate.  Female Ringneck snakes reach about 20cm in the first year-this is a ~60% increase from hatching, reach about 24.5cm in the second year, 29cm in the third year, 34 cm in the fourth year, and approximately 39cm in the fifth year.  Males snakes are slightly larger earlier in development.  They are sexually mature at 3 years.
Hope this helps!