Tinkerspider probably has the first comic.
Technically, Peter could still be 19 if it's set late in the year but Peter's birthday is later (I think it's later been suggested that his birthday is in August, so retroactively that's one of the first stories we could now determine has to be about a Peter Parker who is twenty or older.)
Otherwise the Gwen clone made a comment about Gwen dying two years earlier during the original clone saga, so at that point he's gotta be 20.
He also graduated college in Amazing Spider-Man #181.
I think it's a serious mistake to look into motives for why comics fans or professionals feel a particular way.
People aren't a hivemind. Fans and detractors of the spider-marriage will have different reasons for it, so the suggestion that it's obviously about one thing is likely to be wrong, because what matters to one person may not matter to another.
And it's bad to be obnoxious and wrong. The moment you say something about another person's motives, you've messed up in ways that would disappoint Spider-Man.
I don't think we'll ever know for sure when his "story" would be over.It's a story.
And it's going to end.
The sun is going to end. Life on Earth is going to end. The universe might be infinite, true, but it might also be finite. Still, it's highly highly likely the story will end.
I find it odd, this worrying about a future market. For all we know, storytelling is going to transition to VR and we will experience storytelling in an immersive universe where we walk around and what we see determines what part of the story we experience. Or individually we get to decide the direction of the story for ourselves, so my Spider-Man may look incredibly different to yours.
The market Marvel needs to worry about is the market here, now. You cannot 100% future proof yourself against the market. The future of the market cannot be predicted with any pinpoint accuracy, and Marvel will need to respond if it continues to shift. Maybe Marvel will need to reboot. Or move to a graphic novel format where there is no continuity, just one-off stories that have no relation to the other. Maybe they take a manga route and give the character to a creator who will tell a multi-year saga with a beginning and an end. As it is, there are studies (I think Pew?) that show that Gen Z is less interested in superheroes than Millennials, and far less interested in superheroes than Gen X and Boomers.
I'll also point out again that keeping characters static did not work for DC. Where's Superman's Pal, Jimmy Olsen on the shelves now?
There are three ways of thinking about this.
1. What will be the last adventure of the original Peter Parker? -It's possible that it won't be announced, because sometimes stories seem to end and then continue.
For example, the Earth-1 Superman's story seemed to end with Crisis on Infinite Earths. But then he came back for Infinite Crisis.
2. What will be the last adventure of anyone in the classic Marvel Universe? -Because Peter Parker is part of a larger story, and even if he gets a definitive ending, if the rest of the universe continues, the story's not over.
3. When will be the last adventure of anyone in the wider Marvel multiverse? -Even if the classic Universe ends, there are a lot of different worlds with continuity that touches on Spider-Man like the Ultimate Universe (either the new or old.)
Worrying about a future market is responsible. It's something companies should do, and that anyone engaging in speculation should take into account.
The better argument than ask why worry would be to consider if the market has changed in ways that did not apply decades ago (IE- Back issues are readily available in many digital formats, Miles Morales took off as a character.)