Idiomatic expressions:'他今天成人了'，‘他长成一个大小伙子了’，‘他长大了’

The structure of the sentence "he is a teenager today" is normal in English, but it can't be translated directly into Chinese. You'd better add a word ‘become-成为" when you translate. He is a (became a) teenager today.-他今天成为了一名青少年。 It's my eldest son's birthday today, he's a teenager today!- 今天是我大儿子的生日, 他今天成为了一名青少年。

今天是我大儿子的生日，他今天成了一名青少年。 this is correct but too weird. we will say: 今天是我大儿子的生日，他已经是个大小伙子了。

他已经是一个大小伙子了。 他已经长成了一个大小伙子。 or the newest way people often use nowadays to describe a young boy: 他已经长成了一个小帅哥。(if you think he is still a child) 他已经长成了一个大帅哥。(if you think he is no more a child)

There is no equivalent for teenager in Chinese. This situation is frqquently met when we translate. It's interesting that people (I myself for one)often have an obsession finding the very exact equivalent in one language for a word or phrase in another language. When it proves to be impossible, the translator would still feels undisfactory and sorry. You cannot find a Chinese word for English cousin, and vice versa, you cannot find an English word for Chinese 孝 in Chinese.