When I was a wee young lad of about 12 I was bit by a rattler. It was a young one that had crawled up into the engine compartment of a truck that had been parked for a while. Spent 3 days in the hospital, and the doctor said I was very lucky to not lose any permanent mobility of my hand and fingers, or even lose a hand. As the OP alluded to, the younger ones venom isn't any stronger, but they haven't learned to control it as well as the older ones, so they may hang on and pump more in. As I said, I was fortunate, as this didn't happen to me.
As dwellers many park for up to 2 weeks at a time, plenty of time for something to crawl up inside the engine compartment and make themselves warm. If you're in the habit of checking oil etc... before heading out again, as we all should be, it's a good idea to bag the hood and fenders with your hand for a minute before opening the hood when camped in desert areas. The one that bit me made no sound and I never saw it until after it bit me. In fact, at first I thought I received an electrical shock as I was messing with the fuses and because it was small it's rattle after the bite sounded like an electrical short. Stay safe out there.