Let's go with 20 years per generation. A twenty year old now was born in 1997. that means their parents could have been born in 1977. Which means their parents could have been 20 in 1977. It would not have been odd for a thirty something to go to a concert in the late 80s into the mid 90s.
My wife's mother had her at 17 (not really uncommon in the 70s). My wife didn't get married and start having kids until her late 30s. However, her mother could easily be a great grandmother born in 1960 had my wife had kids in her early 20s.
If several generations had children at 18 years old you could have Great grandmother born in 1960, Grandmother born in 1978, mother born in 1996 and kid born in 2004. Back up a few years and go with 20 years between and it's easy to have Great grandparents born in the mid-late 50s. Great Grandmother born 1955, grandmother born in 1975, mother born in 1995, and kid born in 2015. Nothing unusual about a twenty something mom going to a concert in the mid-late 90s. And she would be the grandparent of a kid today. Have that grandmother going to concerts into her thirties (still not unusual) and there you have it. A grandmother now, born in....say....1965-1970 going to a Matchbox Twenty concert in the 90s.
Fun with family trees and math.