When did rock mature as a genre?

Rock began to mature as a genre in 1964, when the British Invasion came into full swing, and introduced a sound and vibe that was a lot different from that of the earlier years of the '60s. The Kinks hit You Really Got Me would really pioneer the concept of "hard rock", the Animals cover of The House of the Rising Sun was a landmark in that it helped sophisticate the rock genre, and the Beatles A Hard Days Night album would introduce a new sound for the band that had something of a Swinging London vibe to it, and it would be the first Beatles album with entirely original compositions.

However, 1966 was the year the genre had really grown up. 1966 is also the first year I'd consider to be part of the "Swinging Sixties", as it was the year in which public opinion on the Vietnam war became mostly negative, when Swinging London inspired fashion really became popular in the US, and it was when rock truly made the leap from being a fun fad for giddy teenagers, into a more sophisticated art form.
This shift in rock music started with the Beatles album Rubber Soul, which came out in December 1965, and then the shift went into full force all throughout 1966. It was a year that saw the British Invasion begin to be countered by an American wave, and the central pop cultural location moved from the UK to California, with the debut of groups such as the Monkees, the Mamas & the Papas, and most notable the Mothers of Invention with their anti-conformist hit debut album Freak Out!, plus it was when the Beach Boys made a comeback with their legendary album Pet Sounds, which would be a huge breakthrough in the development of "art rock", and would be the inspiration for Sgt. Peppers a year later. Speaking of that, back in the UK, this would be the year the Rolling Stones and the Kinks would begin to shift to a far more sophisticated sound, with albums such as Aftermath from the former, and Face to Face from the latter. The Beatles would of course release Revolver, not only that, but they also quit touring in August of that year, fully establishing themselves as artists rather than performers, plus they shed a lot of their innocent image with John Lennon making the infamous "more popular than Jesus" statement that sparked mass backlash against the band in the US.

I'm not sure if I would really consider 1964 as the year rock matured as a genre, to be honest. I do agree that the advent of the British Invasion spurred a developmental change in the way rock songs were composed, but it was really more of a transitional time for the genre. The notion that rock was in the same state as it was post-1966 is false. The Late '60s saw the development and popularisation of many sub-genres, such as progressive rock, heavy metal and jazz rock. It was at this point that rock music had moved away from it's r&r roots and became an art-form which served as a vehicle for social change.

Rock music in 1964-1965 was still stylistically influenced by r&r/blues from the earlier part of the decade and for the most part, the lyrical themes of the songs still largely revolved around romance and high-school topics.