This is the conversation my friend and I had:
X：你多大年纪
I：我还27岁
X：还？
I：27岁是1990年出生的，跟你同一的90后
X：那应该说，我才27岁
I was going to make an impression that I'm only 27, which implies that I'm not so old yet, even from her stardard. But she said 还 is not the correct word and I should use 才 here.
还 has the meaning such as "still" or "yet".
Why is 还 not correct in this situation?
Although you are trying to say "I'm still 27.", "还" does not work in this context, since it is only used to imply an action is still ongoing or something still exists. Grammatically speaking, it should be used before a verb, not a noun.
On the other hand, "才" in this context is used to emphasize a (merely) small quantity. Therefore, it should be put before a noun. There are other usage of "才", making it can be put before a verb, but that's a different case.
Maybe this example will help you to notice the usage of "还" and "才" (example from Chinese Grammar Wiki).
你才一百斤，还要减肥？
还 has the meaning such as "still" or "yet".
Why is 还 not correct in this situation?
Example: "我的生日是明天，所以凌晨前我還是 (is still) 二十七歲"
Example: "我还未到二十七歲" , "我还未滿廿七歲" or "我还未足二十七歲" (all mean I am not 27 yet)
"我才廿七歲" = "I am only 27" implies it is a young age from your point of view. It is in fact short for "我才(只有-only)二十七歲"
The simplest answer to the question "你多大年纪?" is "我今年二十七歲" or simply "我廿七歲" This answer already indicated you are currently 27, which mean you are still 27, not 28 yet
Edit:
I was going to imply is not if I'm (still) 27 or 28 but if I'm also as young as the friend.
It would be "我也祇是二十七歲"
[我也 (I too)] [祇是 (is only)] [二十七歲 (27 years old)]
No matter your friend is 21 years or 30 years young, the above statement implies you are also young at the age of 27
Nobody says "我还27岁", but you can say "我还 ‘不到’ 27岁 "(I am not 27 yet)."我还 ‘不到’ 21岁, 我不能喝酒 " "我还未到二十七歲" is way to formal.
