Joseph: That's a picture. Of me. Sitting on a couch. With my legs spread open. Naked. This picture was on the cover of GDILFs Magazine's 'Top Ten Sexiest, Most F*** able Grandpas'. I was number one. Beating both Monkey D. Garp from One Piece, and Uncle Iroh from Avatar.
D'Arby: How...recent...is that picture?
In-universe it's mentioned repeatedly that's he's very attractive even in his old age. To the point that the protagonist of the fourth part is his bastard son. Who was conceived when he was 65, after his grandson Jotaro had already been born.
Even taking into account the time he got de-aged by a Reality Warper, Magneto is decidedly not a young man. He was born in the late 1920s, and Comic Book Time doesn't change this since being a Holocaust survivor is central to his character. Chronologically he's in his late 80s, and biologically he's probably in his late 50s or so. Despite this, he's a buff, attractive-looking man whose only real hint at not being young is his white hair...except that it has been white his entire life so that's a false hint.
Sisterhood series by Fern Michaels: Myra Rutledge and Countess Anne "Annie" Ryland de Silva. Both of them are in their 60s and heading on to 70s. Make no mistake, they are old enough to be your grandmother, and they are incredibly sexy! In fact, Weekend Warriors has Nikki Quinn and Barbara Rutledge commenting that Myra looks better at 60 plus years old than they do at 30 plus years old. Now that's hotness!
Belgarath in The Belgariad is an Elderly Immortal who looks to be in his eighties or nineties. Right up until he takes off his shirt, demonstrating that while his face may be that of an old man, his body is not.
Kinsey Millhone definitely thinks her landlord Henry, who is around eighty, is very sexy.
Lady Abigail Irene Garrett in New Amsterdam is around 50 when we meet her, and is definitely still a sexually vibrant woman.
Paul Lassiter's mother in Spin City. Paul is baffled by all the attention she gets until the other guys point out her sexiness, causing Paul to give her the head to toe. "OH MY GOD! I just checked out my own mother!" "And?" "She's a FREAKING HOTTIE!"
Scrubs: JD has sex with a new grandmother (her daughter was a patient, for the delivery), who easily fits the category in "My Best Moment".
Gemma from Sons Of Anarchy, enough to be checked out and commented on by the much younger Prospect, Half-Sack.
Bree van de Kamp from in Desperate Housewives is coming from the Hot Mom territory to this one. Reinforced and lampshaded, in both ways, when, at the same time, she begins her menopause (the grandma side) and gets involved with the handsome, well-build and 15 years younger Keith (the hot side).
Horatio Hornblower: Duchess of Wharfedale who is actually a disguised actress Katherine Cobham from "The Duchess and the Devil". She's very probably not a mother or grandmother, but her age qualifies her, especially for Regency England. Nearly every male character in the story admires her and acknowledges that she is immensely hot and an extremely charming woman.
Dr. Quinn, Medicine Woman: Dorothy was an older and very attractive woman, estranged from her abusive husband and later widowed. She's courted by her brother-in-law Loren Bray and much younger man Jake Slicker, who initiated their May-December Romance. Dorothy's old age is marked when she thinks she's pregnant but she's in fact beginning menopause.