two connections, one for the oldest cultural sites, and one for the newest cultural sites. City WHS present a slight problem, but i would go with time period in which most of the city is constructed, instead of the oldest or newest building.

Oldest
Tsolido - http://www.msnbc.msn.com/id/15970442/from/RS.3/ - 68000 B.C.
Serra da Capivara –some paintings are 25000 years old -23000 B.C.
Qal'at al-Bahrain – 23000 B.C.

Newest
Sydney Opéra House -1973
Ciudad Universitaria de Caracas - 1968
Brasilia -1956-1960

this could possibly be done for natural sites, but it would be harder.

i'm not sure if it can be done for all natural sites though... certainly surtsey will be one of the newest world heritage sites... as for the oldest, not sure... probably those with stromatolites, fossil sites, and even lake baikal... but the problem here is that all natural sites have a long history, but when did they start to have an outstanding universal value will probably be different from that... so that's tricky for natural ones...

We do of course already have a "Connection" for "Built in the 20th Century" which largely covers "newness". I think that the idea of defining "newness" in terms of "when most of the city (site) was constructed" creates some problems with "continuity sites". But equally of course we have to handle living cities which will inevitably will have all sorts of new buildings/structures within them - though UNESCO/ICOMOS try very hard to prevent this from happening. I guess we must limit our interest to buildings/structures which might be considered "significant" in nature, size or location. I would be very interested to know from other "WHS followers" of significant NEW constructions INSIDE inscribed areas which have been "allowed" and also where UNESCO has stopped such developments. I am currently following the debates about some proposed new developments in Edinburgh and Bath with interest

I would include in "Newest" any buildings within a site which are "significant" to its "inscription reason" and the newest I know of is the Qol-Şärif mosque inside the Kazan Kremlin. This was opened on June 24, 2005. The site itself was inscribed in 2000 whilst the mosque was being built. It is certainly "significant" and its impending arrival was recognised in the inscription documentation when ICOMOS stated :-
"The new mosque that is being built within the complex can be understood as new construction in a historic context, where it contributes to the traditional continuity and a balance between the different cultural elements of the place. It should be noted that, considering the character of the site, such a new building should be considered to be strictly exceptional."

The Bahai Gardens at the Shrine of the Bab at Haifa (itself built in 1953) are another "Newest" Candidate - The 19th, and highest, level of the Terraces was officially opened in June 2001. They are clearly a significant part of the inscribed site.

As regards the "Oldest" cultural site - the early Hominid remains sites are inscribed on cultural criteria so I guess that the site of "Lucy" at Lower Valley of the Awash is the oldest at c 4 million years - albeit somewhat debatable as to whether this was "Human Culture"!!

Those interested in "UNESCO Illogicalities" might like to look at Ngorongoro - this is inscribed purely on natural criteria. However, the description on the UNESCO web site mentions nearby Olduvai gorge with its early hominid remains as if it were part of the inscribed site. So do the "on file" evaluation documents - but these are not from ICOMOS or IUCN and go back to 1979 (such early documents are often rather thin and don't follow more modern practice). So the question remains - Is Olduvai Gorge inscribed or not?? Since other Hominid sites utilise Cultural criteria I don't see that it can be. But, in that case, why mention Olduvai at all? The 2008 WHC document clarifying site boundaries doesn't cover Ngorongoro, there isn't a "Locations" tab on the Web site and I don't know of anywhere else where the boundaries and locations are unequivocally defined

well, there's a plan by tanzania to re-nominate the ngorongoro as a cultural site (hence, a mixed site), for sure, to recognize the place for its hominid value. but the process is quite slow, it has been on the list of nominees for a succeeding session, but it's always incomplete, so due to technicality, the advisory bodies could not really evaluate the nomination.

i remember though in the "old" official world heritage website, when they have a separate list of world heritage sites as classified based on tropical forests, urban heritage, paleontological and fossil sites, hominid sites, etc. that ngorongoro, despite indeed being officially a natural sites, is part of the hominid sites listing.