I think he sometimes forgets he’s in the real world now.
He hasn’t left his student life behind.
He doesn’t act his age
– MEANING – he is acting younger than he is, maybe not professional or used to working life.
I think he sometimes forgets he’s in the real world now
– MEANING – the real world is what we use to describe ‘working life’.
Sometimes when we are still a student, we are still living a life with little responsibility.
He hasn’t left his student life behind
– MEANING – he is still acting like and holding onto his student habits.
He doesn’t act his age
– 意味 - 子供っぽい態度をとったり、プロとして／社会人としての意識が薄いこと。
I think he sometimes forgets he’s in the real world now
- 意味 - ‘working life’とは、私たちの社会人としての人生（生活）を意味します。
対照に学生時代は往々にしてあまり責任感を持つ機会も少ないです。
He hasn’t left his student life behind
– 意味 - 彼はまだ学生気分が抜けていない。
Although he is already working, he still seems like a student.
「彼は社会人になっても、学生気分が抜けない」はこのように言えます：
Although he is already working, he still seems like a student.
althoughは「～にもかかわらず・～のに」という意味です。
「社会人」は英語にあまり訳しませんが、(be) workingというのは、「会社に勤めている」の意味となります。
he still seems like a studentは「(行動が)まだ学生のようです」という意味です。
He's working but he hasn't given up his student ways yet
He's working but his day to day behaviour is quite student-like
It seems that you wish to criticise this person for possibly acting a little immaturely...or is it the clothing..or the relaxed attitude that has caught your notice?
Anyway, you may refer to 'his student ways' or his 'student-like' behaviour. If it is something you don't agree with, of course you could use such adjectives as 'juvenile,' 'childlike' or 'immature.'
He's got a responsible job but he still acts like a kid!"
行動がちょっと未熟なのか、服装がそんな感じなのか、またはリラックスした態度が鼻についたのかわからないけど、そういった人を批判したいのかな？いずれにせよ、"his student ways"や"his student-like behaviour" ということができます。それが何か気にいらないことであれば、'juvenile,' 'childlike' or 'immature.'といった形容詞ももちろん使うこともできます。
こういうようにも言えます。
He's got a responsible job but he still acts like a kid!"：責任のある仕事についているのにまだ子供のようなふるまいをする。
He's already working, but his behaviour is still immature.
I think what the question is really saying is that this person has reached "working age' therefore he/she is probably an adult, but still acts like a child. In this case you can say the person's behaviour is immature.
The word "immature" means "childish".
He acts like a student, that's not very professional!
"He acts like a student, that's not very professional!"
Some people do not want to grow up, so, they act childishly and that is not acceptable if you are a working professional. There are a time and place for everything.
"He acts like a studnet, that's not very professional."
いつまでも成長したくない人もいて、子供っぽくふるまい、仕事の場では不適切ですね。常に時と場所をわきまえるべきです。
Although she is working, she behaves like a student.
"Although is used like a conjunction. It means in spite of the fact that or even though.
For example:
"Although the sun was shining it wasn't that warm".
The word behaves mean to act like.
"Althoughは、接続詞です。これは、事実にかかわらずという意味です。
例えば：
Although the sun was shining, it wasn't warm.
太陽が出ているにもかかわらず、温かくない。"
He doesn't act his age even though he's not a student now
Even though he is working he still acts like a student
He acts very immature even though he's working now
To be immature means that you act/do things in a way that make you look younger you could also say 'you don't act your age'
To act means to act what your age is and not younger or in a childish/younger way
”Immature"という言葉は、精神年齢が低い・幼稚なことをしている、という意味として使う形容詞です。”You don't act your age”（年不相応なことするよね）という言い方もあります。
”Act your age"は「年相応に振舞う」という意味です。
He has already graduated from university, but he's still immature.
おっしゃっている内容は以下のようにも表現できると思いましたm(__)m
He has already graduated from university, but he's still immature.
「彼は既に大学を卒業しているが、まだ未熟だ」
graduate「卒業する」
immature「未熟だ」
以上です。
少しでも参考になれば幸いです(*^_^*)
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