A minimum plain bore sprocket is a roller chain sprocket with a hole drilled through the center. A finished bore sprocket is a roller chain sprocket with a hole drilled through the center augmented by either a keyway or set screws.
a flat sprocket that has no hub extension on either side
run a smaller sprocket on the rear
A bore kit does not make a bike faster. It makes the engine more powerful if you want to go faster you must work on gear ratios and or sprocket sizes.
Pipe bore is the actual minimum inside diameter of a pipe, which is not necessarily the nominal pipe size.
88cc big bore kit. thats about it. faster top speed can be achieved with a smaller rear sprocket.
The only thing that I can think of is chainwheels for BMX, or bikes with Ashtabula cranks. These are sometimes called sprockets and can have different diameters for the center mounting hole. The bore would then have to match the axle of your crank for you to be able to use that chainwheel.
There is no minimum or maximum age
a large sprocket
Use full bore for applications where pressure drop should be minimum (such as in the suction pipe of a pump with required NPSH very close to available NPSH), or for pipes which could be cleaned with a pig (so that the pig doesn't get stuck on the valve). In other applications a reduced bore should be OK.
hangfire
the smallest sprocket is the highest gear.
It depends on whether you are talking about the front sprocket (the one at the pedals, called a chainring) or the sprocket at the rear derailleur. The lowest gear at the front is the smallest sprocket/chainring. The lowest gear at the rear is the largest sprocket. So if you combine the smallest sprocket at the front with the largest sprocket in the rear you have the lowest gear available on your bike.