Difficult and requires lots of experience. As a snake ages, its symmetry changes. It's a subtle thing many people even some experts would struggle to tell a four-year from a 14-year old animal. Most 14-year-old snakes are still pretty much in their prime. But, in general, as snakes get older, they tend to get bigger heads, and bigger bodies - not necessarily fatter, though that's usually the case in captivity - but a bigger skeleton, a bigger body cavity because the rib cage is bigger.
Most snakes mature at one or two years of age, and individuals may live up to twenty years in the wild (presumably someone somewhere monitored them from birth..?) but giant anacondas and the black lipped cobra can live up to 30 years.
To get down to specifics for any particular species, it really requires that you know well the symmetry and shape, and thus growth rates of the animals with which you are trying to determine. And that involves looking at lots and lots of them. There's no substitute for experience.