Level 12 is a myth; diamonds are evenly distributed across their entire range, 0-15 (NOT 16, despite what Customized suggests since the game calculates the location of a vein as random.nextInt(max - min) + min; random returns a number from 0 to n-1 (note the "exclusive"), because veins (except emerald) are more than one block (covering up to four vertical layers for diamond) and only generate below their starting point there is less ore above y=12; below y=5 there is less because of bedrock, but if you removed bedrock and they could generate below y=0 there would be the same amount until you reached y=-1 with the lowest veins reaching y=-4.
Also, since the amount of diamond ore falls off above y=12 (about 90% at y=13, 50% at y=14 and just 10% at y=15) and a branch-mine exposes at least four layers (if 2 blocks high) you want to stand at y=11 or below to get the most diamonds (this is equivalent to an eye position of y=12 in older versions, newer versions now use your feet position).
There is a small variance in ore concentration between layers 5-12, but it is insignificant and varies depending on which analysis you look at, generally though there is very slightly more ore near the lower end of the range:
Note that the first chart suggests that iron and coal are less common above y=40 but that is due to including oceans, if you are over land they are just as common up to sea level (iron) and up to y=127 for coal (as a percentage of stone). Much of the decrease in air (caves) is real though. The second chart is a good example of how random noise can cause false conclusions to be drawn; none of those spikes are real and are due to a more limited dataset than the first and last charts (the first chart also suggests that lava is supposedly rarer than diamond below y=11; I think they used a utility like Minecraft Land Generator to create a world, then only looked for stationary lava blocks; prior to 1.7 caves generated with flowing lava which takes some time to convert to stationary lava).