2012 was when the revolutionaries stormed the bastille and overthrew the old guard. But I say it was 2006 when the revolution started, when the manifesto was published: deep NNs can be trained end-to-end, learning their own features [1]. I think this is when "Deep Learning" became a term of art, and the paper has 24k citations. (Interestingly in a talk a Vector Hinton gave two weeks ago he said his paper on deep learning at NIPS 2006 was rejected because they already had one.)

[1] G. E. Hinton and R. R. Salakhutdinov, 2006, Science, Reducing the Dimensionality of Data with Neural Networks