Problem with Petco and other big chain pet stores is that they just sell the snakes to make a profit.
They won't sex the snake because it's a waste of their time and money to do so.
Problem with guessing their age is that depending on the store, the snake could have been malnourished and it's growth might have been stunted. The snake could be older than it looks.
I'm not saying all chain pet stores don't take care of their animals, but I have seen some deplorable conditions at some Petcos and good conditions at others.
My first BP is actually a Petco BP, however it was when the store had first opened and their BP's were fat and happy. Now not so much. I'd rather buy from a reputable dealer and that's what I did with my second.