"It is six years old."
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。"
2020.6.5 Thank you for all the info about Chinese in Taiwan.
Do you know of any good Internet sources for Taiwan Chinese? I know some words have entirely different meanings and pronunciation, but I wasn't aware a lot of neutral endings in mainland Chinese, were pronounced normally such as your example 丈夫 zhang4fu1 ( tw ) vs zhang4fu ( ch )
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'.
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!