When is a bike old?

[Wheelie]

I'm looking into buying a used one sylinder offroader for a trans Africa trip (BMW Dakar, Honda Dominator, etc).

Putting kilometers aside, in general, what would be concidered a reasonably aged bike of the type described above, and for this specific purpose?

Please, spare me the comments like: "any kind of bike will make it". I know, you could probably cross the Sahara on a skateboard with square wheels if you really put your back into it.

In my case, I want the best ride I can get for as little money as possible. I've decided an offroader of about 600 +/- ccm's is the best compromise for me (preferably a one cylindered one). I am looking for a bike which has gone less than 40k kilometers (important with low milage on one cylinder bikes). How old would be reasonable? I know there are no clar cut answers here... Is a bike from the late 80's risky? How about something from the mid 90's?

PS. Any motorvehicle is prohibetively expensive in Norway due to extreme taxes on anything that pollutes. A 12 year old Honda Dominator, one of the cheapest 1990's offroaders around, will set you back about 5000 euros... Please keep this in mind before only suggesting bikes that are less than 3 years old... I need to make some heavy compromises!