As a rule, personally, I'm in the camp of "if it's not explicitly spoken or factually laid out within the work itself then it might as well be fan-fiction," be it coming from the script's liner notes, the writers themselves, critics, the actors, the directors, otherwise I don't know where you could reliably draw the line between the objective work itself and subjective opinion (more on that later).
Indy himself being a morally gray/kinda shitty protagonist is absolutely nothing new in the context of the rest of the movie. Regarding this exchange in particular, "child" could absolutely be interpreted in the context of intellectual or social maturity as opposed to literal age and Indy being a literal pedophile, and no age for either of them is ever given. However, it certainly does not paint Indy as being in the right, whatever unfolded between the two of them, and he gets called out for his shitty attitude by Marion directly. And regardless, as for the audience's culpability, in general I think there's a big difference between appreciating/enjoying a film/piece of art and idolizing/condoning (either in part or as a whole) the creators, individual characters, or the moral stances taken by them.
All this being said, there's a lot of stuff swirling around about the "canon" in the context of this scene, so in the interest of getting the facts out there, whatever they might be, I decided to do some digging myself: *deep breath*
Raiders is set in 1936. While her age is never revealed in the dialogue or otherwise, the original
screenplay as written by Kasdan (search "
INT. "THE RAVEN" SALOON - PATAN, NEPAL - NIGHT") explicitly introduces Marion as 25 in the scene (thus born in 1911), and subtracting 10 years would make her 15 at their last meeting as described in their conversation. That being said I think there's also room to interpret the "10 years" as spoken as potentially being anywhere between 8-12 years seeing as 10 years is a commonly rounded time range. Regardless though, whether she was legally a minor under modern standards, she was definitely implied to be at best borderline when they presumably had their affair. However, NOWHERE in even the Raiders screenplay is Indy's age ever specifically mentioned, but based on the dialogue it's implied that he's at least several years older than she is and it's safe to assume he was past adulthood. It also does not mention anything specifically about what they did together, though again it's implied at least that there was significant emotional involvement on Marion's part. But that's the end of it in the script, and Raiders ends Kasdan's involvement in the series entirely.
In a similar fashion,
Temple of Doom begins in 1935 but with no explicit mention of Indy's age either.
Last Crusade opens on what's almost certainly a teenage Indy though the year of that sequence is never established, only that we rejoin Indy aboard the boat off the Portuguese coast in 1938. And just to be sure, checking the
Crystal Skull screenplay (search "EXT TRAIN STATION DAY"), Mutt, who Marion says is Indy's son, is described as being 20 years old, and so based on the screenplay starting in Nevada, 1957, that places Mutt being born/conceived around 1936-1937, which is during/after the events of Raiders. Still no explicit mention of Indy's age.
Now if you wanna talk about the entirety of Indiana Jones media,
The Young Indiana Jones Chronicles TV show does explicitly establish Indy as having been born on July 1, 1899, which would have made him around his mid twenties to Marion's mid-teens when they were last involved prior to Raiders. I think it's important to note though that this was a series produced by Lucas 8 years after the original movie and with zero credit to Spielberg who directed the original film series or Kasdan who wrote the original Raiders screenplay. This is especially key considering the last, though loosest, bit of metatext.
There is on the internet
a supposed transcript of a Raiders story conference with Spielberg, Lucas, and Kasdan dated January 1978, 6 years prior to the film's release and nearly 2 years prior to the full screenplay drafted
in December 1979. As to its legitimacy, the only mention I've found is in
an interview with Kasdan, where it is proffered to Kasdan who acknowledges its existence though doesn't verify the entirety of its contents. With that in mind, on page 25 of this transcript, in opening discussion about the heretofore unnamed "girl", it's put forth by Kasdan, the eventual screenplay writer, that she and the heretofore unnamed protagonist should have a pre-existing romantic involvement. Lucas, as the original concept creator, is the one who posits making her twenty two during the events of Raiders (in the midst of this Kasdan quips maybe the protagonist would have been 42 at the time) and that it had been 12 years since she and the two had their affair , making her 10-11 and he 30. However, Spielberg, the eventual director, immediately pushes back on this saying "She had better be older than twenty-two." Lucas himself then keeps hemming on the specific age, seemingly settling on 15 as the ceiling of what could be considered "interesting", though he's the only one continuing on about the specific age. While Spielberg does afterward latch onto the idea of her being the one to spur on the initial relationship, I think it's fair to say that Spielberg was not into the idea of the age gap being that large or that early on in her life, and Kasdan, who eventually wrote the screenplay, stays off that specific bit entirely thereafter. I also wanna reiterate this is from an unverified source, and one that still only posits what's basically just a first pass stream-of-thought brainstorming session as opposed to an establishment of canon. This also is to say nothing of the personal interpretations of Karen Allen of Harrison Ford themselves and what they injected into their portrayals of these characters and their backstories.
In the end, then, who "owns" Indiana Jones and gets to say what is or isn't part of the facts of the story beyond what's on-screen or within the work itself? I don't know for sure, but frankly, I think there's a statue of limitations on this kind of behind the scenes/supplemental stuff where honestly I think the responsibility is on the individual to decide for themselves, especially with a near 40-year-old cultural touch stone like this one that now belongs more to the public consciousness than the original creators. I'll say this though: Lucas clearly already had more than a few screws loose long before he splurted out the Star Wars prequels.