Mick Jagger

Mick Jagger, modern patriarch

Mick Jagger, modern patriarch

At this moment I can write and a fact checker can confirm the following shocking sentence: Mick Jagger is a great-grandfather. He got there via the old math, best stated in the way of a word problem. If four mothers (Bianca Jagger, Jerry Hall, Luciana Gimenez and Marsha Hunt) bore Mick seven children (including Jade, Georgia May and James) who yielded four grandchildren, including Jade’s daughter, Assisi Jackson, then who is 2-year-old Ezra Key?

Mick’s Jagger’s great-granddaughter.

I never knew any of my own great-grandparents — not one. A great-grandfather is usually so old that he’s dead. A picture on the...