Mika, my children’s 16-year-old half-sister, is posting old photos on Instagram, group shots with her best friends standing in close proximity to each other, with the simple caption Take me back. Even 16-year-olds are feeling nostalgic. Madeline, my 4-year-old granddaughter, has invented imaginary friends. Madeline’s mother—my daughter Anna—has to call Salia’s mother to ask if Salia can come over.