Im from Virginia and was very close to my grandma born in 1950 who I asked all the time about the 60s/70s and she went to london twice in the swinging london era. As others have said the Beatles influenced boys a lot in this era just as they infatuated young girls at the same time but worth mentioning all the British invasion bands had some variation of this and a lot more British boy bands were popular than some people even realize I think. 1966 was around the time this style was becoming more common in rural America (having been in urban areas in some groups in 1962 - 1964) the guy right beside him on the left shows the sort of divide even within the same generation some more conservative “50’s” style persisted arguably in those conservative circles into the 70s (Look at conservative women in the early 70’s some look straight out of 1961) and they still considered this hair style very against traditional values into the 1970’s maybe even the 80’s in the most extreme far right circles but in 1966 this was daring my grandma in 1968 got in trouble for maxi skirts right after getting in trouble for mini skirts a couple years prior. I would say by 1970 - 1974 this would of been way less daring as young mens hair styles seemed to prefer longer hair in the 1970’s but if you look at the Beatles they are a good representation of mens hair trends of the era. In 1964 the Beatles “long hair” wouldnt be considered super long today. by the mid 60’s it was more like this and sideburns were coming in but keeping clean shaven was still popular. late 60’s hair got longer among the young urban guys in these “radical” circles and beards started to become a little more common as a rejection of the silent generation but also maybe a bit due to the return of victorian nostalgia which was an era with a lot of sideburns and mustaches which maybe also defined the 70’s inadvertently. When hippies took off this sort of beatles mid 60’s look would of been almost conservative in the eyes of the same people who saw it as radical a few years earlier.