I think its really more based on size than age, I don't know what weight you could know for sure though.
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www.freewebs.com/snakesandstuff
"Let us step out into the night and pursue that mighty temptress, adventure." (Albus Dumbledore)
My menagerie 
1.2 Ball Pythons 1.1 Amazon Tree Boa 1.0 Corn Snake (and 9 eggs) 1.0 Dumeril's Boa 2.1.10 Bearded Dragons 2.1.2 Crested Geckos (and 2 eggs!) 1.1.4 Eastern Box Turtles 1.0 Northern Mockingbird 0.3 Chickens 2.0 Cats 1.1 Ferrets plus lots of mice and feeder insects and 5 Black Rat snake eggs 

