If you wanted to express the correct term for someone who is the opposite of old, you would refer to this person as being "young" or "youthful".
Example sentence: She is very young and youthful. She is only twenty two years of age.
「若い人」は "young" や "youthful" と表すことができます。
例文：
She is very young and youthful. She is only twenty two years of age.（彼女は若くてはつらつとしている。彼女はまだ22歳です）
こんにちは。
「若い」は英語で young と言うことができます。
下記は例文ですのでぜひ参考にしてください。
I like working with young people.
若い人たちと仕事するのが楽しいです。
How old are you? 15? You are so young!
君は何歳ですか？15歳？とても若いですね！
Someone young is someone that is considered still able to do many things physically and also someone who still has the energy to think fast and with ease.
There is no specific age that says someone is no longer young as it depends on what is being discussed.
Some other words we can use are:
Juvenile: "That juvenile boy just entered the class, help him find his seat."
Adolescent: "She is still in her adolescent years, be gentle with her."
Teenager: "Teenagers can be difficult to work with since they think they know is all."
Child: "She is just a child, she should not be watching those things on TV."
Kid: "When I was a kid we played outside, climbed trees and had fun without technology."
Fresh: "She is still fresh, she has no wrinkles or any blemishes on her skin."
若い人は、肉体的にまだまだたくさんのことができますし、早く簡単に考えるエネルギーもあります。
話の内容によるので、若くはないという年齢というのは特にありません。
若いを表す他の言葉；
Juvenile（若い、少年の）: "That juvenile boy just entered the class, help him find his seat."（教室に今入った若い男の子の席を探すのを手伝ってあげて。）
Adolescent（青年期の）: "She is still in her adolescent years, be gentle with her."（彼女はまだ思春期なんだ。やさしくしてあげて。）
Teenager（若者）: "Teenagers can be difficult to work with since they think they know is all."（10代の若者は、自分が知っていることが全てだと思っているから難しい。）
Child（子供）: "She is just a child, she should not be watching those things on TV."（彼女はまだ子供だ。テレビでそのようなものを見るべきではない。）
Kid（子供）: "When I was a kid we played outside, climbed trees and had fun without technology."（子供のころ、外で遊んだり、木登りしたり、テクノロジーなしでも楽しかった。）
Fresh（若い、未熟な）: "She is still fresh, she has no wrinkles or any blemishes on her skin."（彼女はまだ若いから、しわもしみもない。）
A juvenile is someone who is an adolescent or a teenager.
It is not a very complimentary thing to say about an older
person, e.g. "He can be so juvenile sometimes!"This means
that the person is childish and immature at times.
A youngster is a relative term that different age groups
would use when referring to someone much younger,
e.g. a 30-year old may refer to an 18-year old as a youngster
and a 50-year old may refer to a 30-year old as a youngster.
Youthful is an adjective which simply means that the person
looks or acts young and vibrant but who may very well be
matured or elderly for that matter.
"juvenile" は「思春期の子」「10代の子」を指します。これは大人を非難するときにも使われます。
例：
"He can be so juvenile sometimes!"
これは「彼は時々子供じみたことをする／未熟である」という意味です。
"youngster" は自分よりもずっと若い人を指します。これは総体的な言葉で、さまざまな年齢の人を指します。例えば、30歳の人は18歳の人を "youngster" と言えます。50歳の人なら30歳の人を "youngster" と言えます。
"youthful" は「〔見た目・行動が〕若い、若々しい」という意味の形容詞です。
"Young."
This is an adjective which means the opposite of old.
"Youngster."
This is a noun and refers to a person who is young. For example, "many youngsters use social media nowadays."
"The youth."
This talks about a group of people who are young. It usually refers to people up until 18 years old but could also be used to talk about young adults up until the age of 30.
"Young."
これは "old"（古い／年を取っている）と反対の意味の形容詞です。
"Youngster."
これは「若い人」を指す名詞です。
例えば：
"Many youngsters use social media nowadays."
（今の若者はSNSを使っている人が多い）
"The youth."
これは若者を集合的に表します。たいていは「18歳までの人」ですが、「30歳まで」を指すこともあります。
"Young" is the opposite of"old". You would refer to this person as "a young person". This could be someone who is very young, like a toddler or child, or it could be someone who is a teenager or a young adult.
For example, someone aged 25 would be considered young by someone who is 60 years old.
Someone aged 5 years old would be considered young by someone who is 9 years old.
Such a person could also be called "youthful". Someone who is old but acts young would be called "young at heart".
"Young" は "Old" の反対の言葉です。「若い人」は "A young person" といいます。これは幼児や子どもを指すこともあれば、10代の人や若い成人を指すこともあります。
例えば、25歳の人は65歳の人からすれば "Young" です。
5歳の子は9歳の子からすれば "Young" です。
「若い人」は "Youthful" と表すこともできます。「年は取っているけど行動は若い人」は "Young at heart"（心が若い）と言えます。
The opposite of old is the word "young" which means someone who is still very youthful like a child. You can use this in a sentence by saying something like "Your daughter is very young still.".
"old" の反意語は "young" です。"young" は「〔例えば子どものように〕若い」という意味です。
文の中では次のように使えます。
"Your daughter is very young still."（あなたの娘さんはまだ若いですよ）
You would like to know what the opposite of "old" is. You want to know what you call a person who is the opposite of "old"? Usually, we would say, 'young' or less frequently, 'juvenile'. Juvenile can mean 'childlike' in a disapproving way - or, it may simply refer to someone who is younger than an adult.
"John is a young Swedish student, visiting the UK for the first time."
"old" の反対の意味の言葉を知りたいということですね。「若い人」の言い方を知りたいと。
これは普通は 'young' です。場合によって 'juvenile' が使われることもあります。
'juvenile' は「子どもっぽい」というネガティブな意味で使われることもあります。単に「子ども」の意味でも使われますが。
"John is a young Swedish student, visiting the UK for the first time."
（ジョンは若いスウェーデン出身の学生です。イギリスを初めて訪れています）
"Young," or, "Youthful," are two adjectives that you can use to describe someone who is not very old and still at the earlier stages of their life.
Example sentences :
- That boy is still very young, he has a lot of life left ahead of him.
- Although he was in his late 20's, Joey still had a youthful face.
"Young" と "Youthful"、どちらも若い人を表すときに使うことができる形容詞です。
例：
- That boy is still very young, he has a lot of life left ahead of him.（あの少年はまだ若い、まだ先は長いよ）
- Although he was in his late 20's, Joey still had a youthful face.（彼は20代後半だったが、まだ幼い顔をしていた）
I personally prefer the word "young" but there are no big differences between all of them. For example - I was young when I finished my uni. He is a freshman.
私は "young" を使うことが多いですが、この三つに大きな違いはありません。
例：
I was young when I finished my uni.（大学を卒業したとき私は若かった）
He is a freshman.（彼は一年生です）
shiroさん
ご質問どうもありがとうございます。
様々な表現があると思いますが、自然な表現を紹介します。
- It's always exciting to work together with young people: I get caught in their enthusiasm and get really motivated myself!
--- get caught in something =～に夢中しちゃう、（良い意味で）～に巻き込まれる
--- enthusiasm = 熱意
ご参考にしていただければ幸いです。
Someonewho is not old can be described as being young, fresh or in their youth. Here are some example sentences:-
1. Young - That is an old picture, I look very young in that picture.
2. Fresh -She is still fresh, she doesn't understand the complexities of life.
3. Youth - In my youth, I was one of the strongest boxers in the country.
「若い人」は 'young' や 'fresh' 'youth' で表すことができます。
1. Young
That is an old picture, I look very young in that picture.
（それは昔の写真だよ。その写真の僕はすごく若いでしょ）
2. Fresh
She is still fresh, she doesn't understand the complexities of life.（彼女はまだ若いから人生の複雑さを理解していない）
3. Youth
In my youth, I was one of the strongest boxers in the country.（若いとき私は国内でトップクラスのボクサーだった）
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