If you check the excerpts of Puer Tea: Ancient Caravans and Urban Chic posted here viewtopic.php?f=20&t=19672&start=15
you can see that in Yunnan people traditionally drink unaged raw puerh or green tea made from puerh varietals. Fermented pu-erh (shu and aged puerh) are more for Hong Kong and Taiwan. So drinking young raw puerh outside of Yunnan may be a new concept it's something that people where puerh is produced have been doing for a long time.
As to which pu-erh are good to drink unaged and raw? I think there's not really any broad generalizations that you can make. There's nothing wrong with drinking unaged raw menghai. Aged and unaged raw puerh taste very different and some people have a preference for one or the other. Aged puerh is less bitter and has fermented flavours. Unaged puerh tastes fresh but is often bitter. I suggest trying as many as you can to see what you like and keep in mind that two teas from the same region are not necessarily the same or even really similar. For example, one raw puerh from Yiwu might have unripe plums as the dominant flavour, another dry cured ham, and a third caramel and cacao.
I think that's because after the PRC was founded, the main Puerh mecca moved from Yiwu to Menghai, those Yin Ji Puerh was mostly plantation blended ones.
I heard the concept 'older the better' started from HK in 50s and advertised by Taiwan later on..because those plantation ones are quite bitter when young and become dramatically changed with a camphor note decades later. During Qing~the ROC, the custom of aging wasn't prevailing as now, and most of Puerh leaves were the small leaf types, none-bitter one.
It's true the Yunnanese traditionally drink young raw Pu..but that's strictly for Gushu types...it's not like you can buy a Dayi Bing and drink right away. Correction welcomed.