David Hajdu, an associate professor of journalism at Columbia University, is the author of Positively Fourth Street: The Lives and Times of Joan Baez, Bob Dylan, Mimi Baez Fariña and Richard Fariña.

BREAK out the guitar-shaped cake pans.

Today is Bob Dylan’s 70th birthday, an occasion that essayists, bloggers and magazine writers have been celebrating for weeks. Mr. Dylan surely deserves the attention, but he’s only one in a surprisingly large group of major pop-music artists born around the same time.

John Lennon would have turned 70 last October; Joan Baez had her 70th birthday in January; Paul Simon and George Clinton will reach 70 before the end of this year. Next year, the club of legendary pop septuagenarians will grow to include Paul McCartney, Aretha Franklin, Carole King, Brian Wilson and Lou Reed. Jimi Hendrix and Jerry Garcia would have also been 70 in 2012.

Perhaps this wave of 70th birthdays is mere coincidence. There are, after all, lots of notable people of all ages. But I suspect that the explanation for this striking cluster of musical talent lies in a critical fact of biography: all those artists turned 14 around 1955 and 1956, when rock ’n’ roll was first erupting. Those 14th birthdays were the truly historic ones.

Fourteen is a formative age, especially for people growing up in social contexts framed by pop culture. You’re in the ninth grade, confronting the tyrannies of sex and adulthood, struggling to figure out what kind of adult you’d like to be, and you turn to the cultural products most important in your day as sources of cool — the capital of young life....