There are two types of seller in China. Former is 'drink new Sheng it's ok', latter is 'Sheng at least need 10 yrs of storage before trying it'.
I think the reason of latter might apply to specific tea tree called 大葉種 - Da Ye Zhong, Big Leaf Variety. Which not only turns out bitter/astringent at first, but also could lead to stomach cramp if drinking too much of it (I've noticed ppl in West might have a slight different organ system receiving)
There are hundreds of wild varieties in Yunnan, and most of em actually come hazardous to body before thinking about trying. Leaves used for Puerh are actually selected ones out of them, which is smoothest and tasty that might have been good for a beverage.
Here's a quote written by Jiang Yu Fa
"The term 'Heicha' was first used in 15th century script Ming Shi 明史, from Cha Fa 茶法. "Xiangcha (湘茶, 湘 was another name for Hunan) was low-quality so it was made black and became Heicha". That is, Hunan, Yunnan or Sichuan-grown wild trees were so bitter that it required natural-fermentation for a long time to be drunk. After these leaves natural-oxidized and turned black, so 'inner' people called it Heicha. Based on this fact, today's China's all post-fermented teas are called Heicha"
15th century, so the romour hasn't been roaming around just recently.