Only the breeder has an idea on the age, I have BP hatching out at 90 grams just like I have had some that were 3 or months and were around 80 grams.
Weight is not a way to determine age, depends on how big the snake is out of the egg (it can be 45 grams like it can be 90 grams), how fast the animal eats own it's own (Some will after their first shed some will take 6 to 8 weeks), feeding frequency, prey size etc.
As far as what to feed a weaned to small adult mouse will do for the rest see the feeding chart in the husbandry forum.
As for contacting Steve try the reptile collectives's Facebook, he answers pretty quickly. https://m.facebook.com/ReptileCollective/