I think they are adults in typical chibi Japanese design, but I think they made characters cute to appeal to children as well. The characters are living out on their own so there's that. However, I think the ambiguity is meant to create a sandbox environment so Animal Crossing is a game anyone can play, regardless of age. The beauty of Animal Crossing is that you can decide whether or not to roleplay whatever you want. If your OC is an adult, then it is. If your OC is a child, then it is. It is entirely up to the player.
I voted for adult because you’re living on your own and Tom nook forces you to work for a little money in the beginning prior to new leaf.
However, there are some examples of villagers speaking to you, as of a child, particularly in City Folk.
I was recently, watching a birthday video, and one of the cranky villagers, said something along the lines of “you are one step closer to adulthood”
And I think I can very vaguely remember back when I was in school, one of my normal villagers, asking me if I was still on summer break. I think it was my last day before the first day of school. I think I was about to go to 8th or ninth 9th grade…?
And throughout multiple installments, cranky villagers, call you “kid” or “kiddo”. However, this could just be a term of endearment, since they tend to be the oldest of the villagers.
Neither, I think it's left ambiguous so that the player can identify with their character, or imagine them to be whatever age they want. The art style can go either way, really; if you're a child then your character already looks like you to a degree, and if you're older it's easy to just consider everyone in-game as being kind of chibi.
And while the player's situation (living on your own, paying off a home loan) can read as being an adult one, it's also not uncommon for games directed mainly at kids to give them more freedoms than they'd realistically have (coughcoughPokémoncough), and the setting of Animal Crossing is pretty fantastical already.
Basically, I think the player character's age is whatever you want it to be.
To be honest, I see all of the characters present as both adults and children, and I have a bunch of villager and special character head cannons, which specify their ages.
And as for the player, I think the player should be the same age as you, if you are playing as yourself, and side characters not meant to represent the person that created the side characters in real life can be represented as original characters, I think their ages are really up to the player that's meant to represent you.
I've always thought of the characters as adults and to me that just makes the most sense. Even when I was a child playing the game back in the day, I liked playing more of an adult character and having my own house and freedoms. To me it always felt different to how kids can be treated in other games and I think that's part of what makes the game good for both kid and adult fans.
I've always seen the players as children, and considering this is a fantasy/fictional world with talking anthropomorphic animals, I think it's not that odd for someone that young to be moving out on their own. Maybe in the later games they've grown up more to be young adults or teenagers, and that's why their proportions are different.
I think the villagers are adults. I’ve just always seen the player as my own age though. They seem like a young adult maybe, just moving away from home
The villagers and the player get to live in their own houses and they can drink coffee and the player can pay off debt. I'm absolutely sure they're adults.
I think certain personalities, (for example, cranky, snooty, smug) are definitely adults. But I think they all are adults, but some are just kind of younger adults. I think peppy and jock villagers are probably younger and cranky villagers would be older.
Mostly adults, but I see some of them as kids. The ducks all feel like kids or teens to me for some reason. Scoot, Joey, and Deena are little kids but other ducks like Mallary feel like teens.
