You can't tell age on a ball python by length any more then you can tell age on a person by their height. I've got some old adults that are just past the 2 1/2 foot mark, and I've had some that have gotten longer then 3 feet in a year. At 27" 270 grams seems a little light to me though. However young snakes will often put on a growth spurt where they will lengthen out while staying thin then stop and start putting on weight.