It is more than a stretch to believe that Susan Sarandon is old enough to be Melissa McCarthy's grandma. They did a solid job with Sarandon's hair and wardrobe to age her but it was not enough to suspend one's disbelief. With Allison Janney as Tammy's mom, it seemed to shed even more light on the ages being implausible. Sarandon is only thirteen years older than Janney, Janney only eleven years older than McCarthy. The ages are so off, it almost suggests that it was done on purpose, like some kind of Bad Grandpa routine, though I am still missing why.
Tammy was written by McCarthy and her husband, Ben Falcone, who also directed. Falcone often cameos in McCarthy's films, most memorably in Bridesmaids and also in the Heat and Identity Thief. Will Ferrell and Adam McKay also produced.