"It is six years old."
I was reading that this form of 它(tā) is not commonly used. Can anyone elaborate?
For example, Chinese doesn't use it (它 tā) for a "dummy subject".
Its raining = 下雨呢 。
... But if the subject refers to a real thing you'd rather keep it. (Just an addition :)
它 basically represents animal, so not commonly used. for other things without lives, We don't use 它， we use 这（this）or 那（that）
它 is generally used for an animal, something that doesn't have a gender or the gender is uncertain. It should be uncommon to use '它' because you don't talk about animals very often. If you are talking about your pet or something else that you know the gender of, you would use '他‘ or '她'.
In Taiwan "它" is further divided into two characters: "牠" and "它". We use "牠" for animals and "它" for nouns which are not people or animals. (Wiktionary says this character is archaic but it's still commonly used in Taiwan.)
Therefore the Taiwan version of this sentence is "牠六歲。"
You don't use the verb 是 here. This is confusing when you go from English to Chinese, because we use the "to be" verb for so many things that Chinese doesn't. For example, the question "How old are you?" is 你几岁了 (Nĭ jí sùi le), so you should answer the question by swapping out the question words for the answer bits. 我二十九岁 means "I'm(我) 29 (二十九) years old (岁)."
Always remember that because English and Chinese come from such drastically different roots, that a lot our the language conventions from English will absolutely NOT carry over. Be especially careful with your use of 是.
Hope that helped!
is 它六年岁 wrong, because 岁 already contains "years" in it ? then, how to say "he is 6 years and a half old" (not sure this is correct English either, but I hope you will see what I mean)
The 年 is unnecessary. It is implied with 岁 which functions as a verb （though its really a measure word for years!) to be a certain number of years old. Six and a half years old would simply be 它六岁半