Is this the biggest age gap between romantic leads where the actress is the older?

Harold and Maude would be likeliest.

Admittedly a comedy but there is a romantic element.

Starring Bud Cort (born 1948) and Ruth Gordon (born 1896) a 52 year difference

Sextette (1977) tells the story of Sir Michael Barrington (Timothy Dalton , born 21 March 1946), the sixth husband of Marlo Manners (Mae West, born August 17, 1893). This makes for a difference of a little over 53 years.

Reviewer 'Leofwine Draca' notes that West's age is never referred to in the script.

SEXTETTE is a throwback to the early years of cinema, where stars were larger than life and films threw in song and dance routines at regular intervals in the name of popular entertainment. It's also a vehicle for elderly starlet Mae West, at the tail-end of her career, playing a man-eater celebrating her wedding to a young toyboy (a visibly embarrassed Timothy Dalton). [...]

Clearly the director is in love with West and the film has been concocted as a tribute to her. But the format is very odd and old fashioned and West's appearance is rather shocking; her age is never mentioned so it's pretty weird to see her trying to play herself some fifty years previously.

In Adore (2013), Naomi Watts plays a mother who has an affair with her best friend’s 20 year old son, played by James Frecheville, who is 23 years younger than Watts.

For other women more than 16 years older than their male romantic leads, see: https://www.zimbio.com/The+Most+Uncomfortable+Age+Gaps+in+Movies

In Moonstruck (1987), there is no discussion about the characters' ages. Nicolas Cage (born January 7th, 1964), who plays Ronny, is 17 years younger than Cher (born May 20th, 1946), who plays Loretta.

As Roger Ebert summarises:

And at the heart of the story, there is Cher's astonishing discovery that she is still capable of love. As the movie opens, she becomes engaged to Mr. Johnny Cammareri (Danny Aiello), not so much out of love as out of weariness. But after he flies to Sicily to be at the bedside of his dying mother, she goes to talk to Mr. Johnny's estranged younger brother (Nicolas Cage), and is thunderstruck when they are drawn almost instantly into a passionate embrace.

I guess Ronny is referred to as Johnny's younger brother, and one might infer that Johnny and Loretta are close in age, but there is no suggestion that there is a large age difference between Ronny and Johnny.

In American Pie, Eddie Kaye Thomas (October 1980) plays Paul Finch who has a romantic relationship with Stifler's mom, played by Jennifer Coolidge (August 1961), which is a difference of just over 19 years.

Film Stars Don't Die In Liverpool tells the true story of ageing Hollywood superstar Gloria Grahame's affair with a much younger man.?Gloria was played by Annette Bening (aged about 59), and her lover by Jamie Bell (aged about 31), a difference of 28 years — which is also the age difference between the characters they portray.

Reviewer Susan Wloszczyna notes:

It is somewhat refreshing to witness a May-December romance from an older female perspective and both leads pour their hearts into their roles.

Sunset Boulevard (1950) is one of the earliest movies I can think of where the actress was significantly older than he actor with there being a romantic plot element between the two.

Gloria Swanson - 1899

William Holden - 1918

For a difference of 19 years.