There is a significant body of compelling research indicating that the size of the rattlesnake, and therefore the quantity of venom injected, is the most important determining factor of the severity of an envenomation. Bigger snakes tend to cause worse envenomations.

An adult rattlesnake produces, stores, and injects anywhere from 20–50x more venom (more sometimes, but let’s play this conservatively) than a baby. In this case, the huge increase in total quantity of venom injected has a much greater clinical effect on humans.

It is therefore safe to say that the overwhelming majority of the time, adult rattlesnakes produce more serious envenomations than their younger counterparts.

The only thing it says it’s that a baby snake can also kill you and that an snake bite is a snake bite, but as it says in the last quote: Adults are more dangerous that youngs.