Without already knowing, I don't think there's a sure way to tell. All pythons grow at different rates, and you're snake could be anywhere from 5 months to a year old. I have three snakes that hatched out this year, and they weigh in at around 100, 150, and 200 respectively, and I don't have a hatch date for any of them. My 2013 of unknown month is almost 600, and I'm thinking my 800g boy is around 2 years or so, but I have no actual idea. Don't let it bother you too much, it isn't very important to know as far as husbandry goes.
My best guess would be around 6 to 10 months old, but the possibility of being older is there.