So I'll first preferace that the general acceptable lower bound age range of the Younger partner (Y) is usually understood to be Y = (X/2) + 7 where X is the age of the older partner, or better said as Half Age plus 7. This means that, as pointed out, the older the older partner is, the younger the younger partner can be relatively. For example, a 30 year old would look odd with anyone younger than 22, but an 80 under this logic should not be odd with anyone as young as 47 (it's rarely treated this way, as the younger is seen as a gold digger and the older is seen as a dirty old man/couger/cradle robber). Under this model, the youngest acceptable age difference of is a 44 year old dating a 29 year old (exactly 15 years junior... beyond 44, the age tends to rise).
That said, wide age gaps in relationships are not unheard of and the junior may seem naive and the older might typically seem predatory in fiction (heaven forbid that someone actually be attracted to much older partners and prefer them to their peers!). There are some cutures where it's a bit more common (royalty back in the day, or other arranged marriages, the LGBT community (especially among men) more commonly have wider age ranges because of the small dating pool. And as said, some people are more mature than others.
It's also not uncommon if one actor is playing a much younger character and starts dating a co-actor playing an older character. The two might be closer to age in real life than their characters and while off putting to fans (especially with the bleed through in their more tender moments) it's wrong... just gotta keep reminding themselves of that. On the TV show "Heroes" for example, the actors playing the characters of Peter and Claire did start dating each other, even though the characters they played where a man in his late 20s and a high school girl (Sophmore age, I think)... and an Uncle and Niece respectively (Fans were more put off by the romance bleeding through and making the love somewhat incestuous rather than peodophilic... though neither was without comment and they had a lot of scenes that hinted at the real life relationship.).
If there is not a romantic relationship, strong intergenerational friendships are much more encouraged. The character of Wolverine has had a long history of having a teenage to early 20s aged sidekick from the X-men's pool of characters, when he's clearly in his late 40s (and is really older than that). Kitty Pryde, Jubilee, Rogue, and X-23 are some examples of characters he has a sort of paternal bond with that shows they clearly care about each other, but aren't romantic... depending on the age of the younger character, they can even milk the situation of meeting the dad when taking the daughter out and it's Wolverine. It also humanizes Wolverine by giving him some emotional connections (he also seems to get paired up with the worlds worst student driver for added humor).
Similarly, in Buffy the Vampire Slayer, the titular character is a High Schooler initially dating Angel, a Vampire who has been alive for 200 some years, and while still young looking, he's clearly older than high school ages. Rather than ignore the problem, they steer into the skid and address the issues about the age difference in several episodes to the point that it became a major element of the second half of season 2 and was the principal motivation for their eventual break up (and the fact that he was getting a spin-off series).
A more serious example would be the film Gran Torino, in which Clint Eastwood's character teaches a young neighborhood boy of about 14 to be a man. In this case, Eastwood is steering the boy away from joining a gang by teaching him about proper work and instilling a work ethic in the kid. Eastwood's character also is quite friendly with the kid's college age sister, who snarks back at Eastwood's racism against... everyone... which impresses Eastwood and when the local gang hurt her, he takes it as if she was his own family (and even earlier notes he has more in common with them than his own grown kids and their families).
Another rule of fiction is again, since they are not the heroes, but anti-heroes, which generally can get away with a little more. To me, it sounds like it could be very well done if it's made clear early on that their relationship is very loving and respectful. To me, I would almost make it akin to the Fisk/Vanessa paring in the Netflix Daredevil series, where its clearly an awkward relationship, but beneath all the uncomfortable behaviors, they really do find each other attractive and have eyes only for them.
Another similar example is the relationship between Roger and Jessica Rabbit in "Who Framed Roger Rabbit?" All the humans cannot believe a hot girl like Jessica would date a goof like Roger and all the Toons can't believe a Stud like Roger would just settle for a nobody like Jessica (Eddie (human, upon seeing Jessica): disbelieving tone She's married to Rodger Rabbit? Betty-Boop: wishing tone and completely serious Yeah! What a lucky girl!) and because of this both the Toons and the humans see one side of the relationship as some sort of sham marriage. Truth is, they do love each other very much and just have a bizarre way of showing it (Jessica does believe that hitting Rodger over the head with a Frying Pan and throwing them into the trunk of the car would keep him safe (Toon Logic is a well established Oxymoron at this point in the film). And both separately get violent with Eddie when he questions either one's commitment to the relationship they have to one another... You don't have to understand why it works to understand that it does work.
Let the age questions be open questions. Have characters even question the nature of a 15 year age gap... but never let the characters question either member's commitment to the relationship and their feelings for each other without an immediate rebuke along the lines of it works, even if the world says it should not. And that's good enough for them and should be good enough an answer to the naysayers.