So you think the song says little boys soon lose interest in marijuana?
The song is based on Ogden Nash's 1936 "The Tale of Custard Dragon." A little girl had a dog, a kitten, a mouse, and a dragon. The dog, the kitten, and the mouse boasted that they were brave and ridiculed the dragon, who considered himself cowardly. When a pirate with six guns burst in, everyone but the dragon panicked. The dragon became fierce and ate the pirate. The danger past, he again became timid and the others boastful. It's like the way John Wayne, notorious draft dodger, became America's fighting man through movies designed to denigrate those who actually fought.
Leonard Lipton, a college student born in 1940, revised the poem to "Puff the Magic Dragon" in 1959. He promptly forgot about it, but Peter Yarrow, born in 1938, came across it, wrote a tune, and recorded it with Paul and Mary. It was a hit, and he set out to find out who Lipton was so he could share royalties.
Both denied that the lyrics had anything to do with drugs. Indeed, they don't make sense that way. Both were small children in WWII, when America was in danger. As "Jackie Paper," they could read about heroic Americans in the newspaper. Puff: gone in a cloud of smoke: by 1959, college students were indifferent to the heroes who had saved America. Their interest focused on personal advantage.
Then came the Boomers, lacking the attention span to listen to the whole song before asserting what it was about.