Just so you know: You can actually cause internal injuries (even fatal ones) by stretching a snake's body to measure the length. (Snakes have been injured & killed by law enforcement cluelessly doing that when they measured snakes they confiscated.) Even when being gentle about it- it's easy to under-estimate our strength when incorrectly applied to a snake's body- and it's just not that important to have an accurate length measurement of a snake anyway.
I know of no such charts- young & growing snakes tend to look on the thin side for several reasons:
1. They're eating young rodents (usually) that are more digestible & have more water with less minerals (smaller undeveloped bones) & other nutrients.
2. When snakes grow, they put on tiny amounts of length, so any gain to their body thickness doesn't show up. You just need to be patient...
If a young snake is eating regularly, don't lose any sleep over measurements.
Likewise, guessing the age of a snake is pointless, because snakes grow according to how much they're fed, & how much they can digest (this is totally temperature dependent). FYI, some snake breeders deliberately minimize the growth of their hatchlings by "maintenance feeding"- something I don't agree with & would never do, but many others do. They don't want them outgrowing their minimal housing while waiting to sell them- plus many buyers want cute little ones that aren't yet scary biters. So that makes it very hard to know a snake's age unless the source is honest. With wild snakes, there's no telling as few of them have as much to eat as our captives, & many have to fast through cold seasons.
All I can say is that a 20" BP is probably at least a year old.
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