Shog, this idea is actually tossed around a lot
. I've heard from many people that they think young sheng is harsh on their health. They talk about stomach upsets and the like. I've also heard that in traditional Chinese medicine, young sheng is bad for one of the following types of people: Warm-Wet
. It's come up in quite a few threads on here, and on the Puerh livejournal community. Research time for you, my friend.
For me, it's much simpler... drinking much young sheng not only is not pleasing to my taste buds, but tends to hurt my stomach; more so than drinking other types of tea. Could be the caffeine; could be the qi; could be adulterants in the tea.. I don't really care, but the end result is that I don't like drinking a lot of young, or even adolescent, sheng.
Of course, there are plenty of people who drink it all the time, or have no problems with it. I don't know if it's because they're a different type, or what.
I think your body will (usually) let you know if you're doing something that doesn't agree with it.