Don’t want to be chan

I just started a new job in Japan.

Something about me makes others assume they can automatically call me Maiko-chan. I'm 30 years old, but perhaps I look like I just graduated from college... I speak Japanese in the office.

Other than the company boss, the others use teineigo with me, but I still am called "chan". All of it seems normal, except, why the "chan" part?? My worry is that I'll begin working in the office as a child than as a 30 year old adult.

This isn't the first time I've been automatically been called "-chan" without asking first. My husband has Japanese acquaintances/clients whom I've never met. He tells me they ask him, "How is Maiko-chan doing?" or "Maiko-chan should join us for dinner!" He tells them to call me "san" instead.

Other times were when I was in a leading role in the office, but those under me, although older than me, still called me "Maiko-chan," as well as my equals, whom I never gave permission to call me that.

I'm Japanese-American, and I'm not sure how to address this. Should I ask to call me "Maiko-san" or even just "Maiko"? Should I ask them to use my last name, "Sato-san?" How should I ask? (You can write in Japanese, it's ok) I'm worried that this use of my name will let them think they can treat me as a child. What do you think? Maybe I'm thinking about it too much?

If you prefer to be called by your last name, then the next time someone calls you Maiko-chan politely tell him to call you by your last name. But you don't get to choose -san (様) or -kun (君), that part is up to the corporate culture (some companies call everybody -kun, male and female).
Example: 佐藤です or 佐藤と呼んで下さい、お願いします。

Same thing when you introduce yourself, just mention your last name only. Like: 佐藤です、よろしくお願いします。

It should be noted that -chan is not reserved for children and young people, in case you feel that you are too old for that.

"chan" doesn't always mean childish. Despite you are 30 yrs old, your firm boss or someone older than you often call you "chan" because they fell you are a closer person, not close as friends but you are a familiar person.

I know a Ms. Maiko about your age. She's a very stylish and sexy young lady, and yet I must confess that there's something about that name that suits -chan. I guess it just sounds right, as a sound, I mean, and not as a meaning. I think they mean no harm nor are they treating you as a child. At least I'm not. It's almost like a common nickname for Maiko. But you can just say "Sato de onegaishimasu" and they'd automatically call you "Sato-san" just like a 30 something adult should be called with no affection whatsoever.

(You can write in Japanese, it's ok)

では、遠慮なく。

それは、大きな誤解(認識不足)に過ぎません。
敬称である「ちゃん」は、子供以外にも普通に使っていい(構わない)のです。
https://ja.wikipedia.org/wiki/%E6%95%AC%E7%A7%B0
https://en.wikipedia.org/wiki/Japanese_honorifics
上記リンク内にも書いていますが、もう少し判り易く噛み砕いて書くと、
『大人・子供に関係(区別)なく、親しい友人や仕事などの同僚間でも使われる。』と言うだけの事。
年上への使用例として挙がっている「おじいちゃん、おばあちゃん」以外にも、
おじちゃん(おじさん)、おばちゃん(おばさん)、お兄ちゃん(お兄さん)、お姉ちゃん(お姉さん)も在るし、
もっと砕けた「兄(あん)ちゃん、姉(ねえ)ちゃん、お嬢ちゃん(お嬢さん)」なども同じ理屈です。

貴女(あなた)より年上の方から「ちゃん付け(づけ)」で呼ばれる事は普通ですし、
会社(仕事)の仲間からそう呼ばれる事も特に不自然では有りません。(※年下は除く)
むしろ、毎回「さん付け」で呼ぶ方が堅苦しい(張り詰めた・緊張感が強い)空気になるので、
具体的な職場環境までは判らないけど、それで円滑に回っているなら至って正常です。

女性の場合には、「さん」か「ちゃん」が似合うどうかは、
「見た目」「性格・言動(言葉使い)」「名前(音感)」によって決まる事が殆どで、
「大人っぽい人」や「背の高い」人が「さん」と呼ばれ易いキャラで有り、
「かわいい・若く見える」か「小柄」で有れば「ちゃん」キャラになり易いです。
「ちゃん」は「若さ」の象徴でも有るので、決して「大人扱い」していない訳ではないです。

「佐藤」 or 「佐藤さん」？
佐藤の苗字(Last name)自体が男女問わず多過ぎるので(日本で 1番)、
下の名前(First name)か、あだ名(nick)を付けて区別する事は普通です。
http://www2s.biglobe.ne.jp/~suzakihp/ju0001.html
それだけ呼び間違いで混同する事は、仕事上かどうかに関係なく
色々と問題が生まれ易くなってしまうので、好ましくは有りません。
なので、もし職場に男女問わず貴女以外の「佐藤さん」が居ないのなら、
そう呼んで貰う様にお願いしても別に構わないとは思いますが、
あまりにも普通の名前で有る事には変わりがないので、
下の名前が覚え易い(or特徴的だ)と上手くなじまないかもしれません。

それと、「マイコさん」の場合、「舞妓さん(京都。芸者見習い)」と同じ音なので、
「マイコちゃん」の方が「一(いち)個人」を特定する事には適しているとも言えます。
https://ja.wikipedia.org/wiki/%E8%88%9E%E5%A6%93

会社の親しい同僚に、「何故、私は『ちゃん付け』なの？」と訊いてみて下さい。

I am a Japanese male of old generation.
I don't think that calling somebody with -chan in ordinary working places is an ordinary behavior. I know, however, that in some places they call each other with -chan in order to express their feeling of closeness, of certain familiarity, of equals, or as simple habit of the working place, but I say again that in ordinary places it is not normal. Women tend to call each other -chan; is your office something like that?

In the first place, I wonder what made your fellow workers and your husband's aquaintances "assume they" could "automatically call" you "Maiko-chan". As simple impression, considering that your husband's aquaintances call you automatically -chan although your husband tells them to call you -san, I think that, in their mind, Maiko-chan is tightly linked as a unity rather than because they chose -chan as suffix.

You say you are Japanese-American. I imagine that you are in their mind a foreigner speaking English with whom they feel familiar because of your half Japanese looks. In these cases, psychologically analysing, Japanese people tend to think or misunderstand that they are your close friends by calling you frankly with first name, imitating, with unconcious feeling of satisfaction, American or Western way of first-name based relationship.

I don't know that they are naive enough to call you -chan with no consideration, or that they are lacking common sense, delicacy, and imposing an excess of their one-sided familiarity. But, the important thing is that you don't like to be called -chan in your working circumstance and you want to draw a line between your public and private life, you don't need to hesitate to ask them, firmly but in a friendly tone,
"あのー、ごめんなさいね。まいこちゃん　じゃなくて、佐藤さんって言ってくれませんか？"
”それとも、わたしも、みんなを　〜ちゃん　って呼んだ方がいいのかな？”
That is a reciprocity. And as a consequence, they might get aware of unusualness and strangeness to call you -chan and to be called by you -chan, all using "teineigo".

If, nevertheless, they show wondering attitude and continue to call you -chan, you should tell them simply and firmly to not call you -chan anymore.
〜ちゃん　って　呼ぶのは　これからは　やめてください。

Thanks for your answers, everyone.

I also consulted my Japanese friends on this and they tend to agree that I won't be treated like a child, which is my biggest worry.

I realize I have the same name as a 舞妓, and usually they add -san to those girls to make it sound like a profession. Also, since I'm not culturally Japanese, it's easy for them to assume to call me by my first name, even though I'm speaking Japanese.

I think as an American, the idea of being a child is not great and growing up we want to be adults asap. We don't like being little. I know in Japan, many people adore children, and some of my friends in college didn't want to graduate.

I guess it feels strange to be automatically called -chan, when I don't know the person that well yet. However, "..." raises a good point. I should ask, "Should I call everyone in the office chan too?" Might be interesting to see what happens...

Ultimately, I'm an American in Japan, so it might be better for me to get over it.