1. His age is 14.

Question

2. His height is 158cm tall.
3. His weight is 48kg.

(Are the sentences all grammatical?)

4. What is the negative sentence of the declarative sentence? When you make a negative sentence, you should put 'don't' before a base verb.
(Is this one grammatical? Would you check it? Thank you.)

Answer 1

1 and 3 are fine.
In 2, you don't need "tall" because the word "height" covers it.

4 looks good except that the word "sentence" is repeated. You can simply remove the first instance of it, or you can write something like this:

How do you make a negative in a declarative sentence?