Many snakes experience an ontogenetic shift in diet, for example, eating lizards when young and mice when older. A number of studies have found that snake venom composition and taxa-specific toxicity can also change as snakes age/grow, probably due to the shift in diet. For example, young brown tree snakes on Guam eat lizards and the adult snakes eat birds. This change in diet is accompanied by a change in the toxicity of their venom, with the venom of young snakes being more toxic to lizards than that of adult snakes.
Mackessy, Stephen P., Sixberry, Nicole, M., Heyborne, William H., and Fritts, Thomas (2006) Venom of the Brown Treesnake, Boiga irregularis: Ontogenetic shifts and taxa-specific toxicity. Toxicon, 47:537-548
To summarise: I'm not aware of any studies that have shown that juvenile snake venom is less toxic than adult venom, but it wouldn't surprise me if that situation occurred in some species out there. There are studies that show that the juveniles of certain snake species have venom that is more toxic than that of the adults.
I should say that all the studies I've read have made no distinction between age and size, so you don't really know if venom changes as a snake ages or as it grows.
I was going to look at this field as part of my PhD studies until the above article was published in 2006. Stuffed my plans up completely. Not that I'm bitter about it.
Stewart