You can't determine age based on weight and even a low weight is not always the sign of maintenance feeding.
Some animals smaller than other naturally (genetics can play a huge part), some that are picky and will skip meals or fast, etc…there can be various reason for weight discrepancies which is why people need to rely less on numbers and more on overall condition of the animal
And it all start as hatchling some are 45 grams some are 90 grams, some will get started 1 day after their first shed some will take their first meal 6 weeks after their first shed.
That is why when buying you buy from a reputable breeder that knows the hatch date and can give you clues on the animals feeding history.
Over the years I have had BP that I hatched and raised that were 350 grams at a year and some that were 1200 grams at a year and everything is fed aproprietly once a week.