Yes, it's a myth, with various versions.
The venoms are sometimes a little bit different between juveniles and adults. A typical situation is the venom of juveniles being a little more effective against lizards/frogs and the venom of adults being more effective against birds/mammals, to suit the typical prey choice at different stages of life, but generally this is minor and would make juveniles less venomous to humans, along with the far lower venom yields which make juveniles much less dangerous. There are also some silly versions which say juveniles inject more venom by choice or because they can't control their bodies etc, which are all baseless nonsense. Herpers love a good story and if they don't have one they'll just make something up, and the others will be eager to believe it.