It can also depend if your bp is male or female. Females get thicker, but I dunno when they start to show a noticeable size difference.
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I dunno about average, but Pelota is currently 5 months old(6 on December 8th...ooo..already half a year) and she's about the thickness of a banana, and about 23-24 inches(havent measured her recently).
2.0 python regius - Ace(pastel) and Pelota(cross-dresser )