1. Introduction:

Punctuation is important in writing because it helps to convey meaning and clarity to the reader. In the sentence "My sister is 5 years old," we need to use the correct punctuation to ensure that the sentence is clear and grammatically correct.

2. Comma Usage:

When writing a sentence with an age, it is important to use a comma between the subject and the age. In this case, the subject is "My sister" and the age is "5 years old." Therefore, the correct way to punctuate the sentence is: "My sister is 5 years old."

3. No additional punctuation needed:

Since the sentence is a simple statement of fact, no additional punctuation is required. Avoid using any other punctuation marks such as periods, exclamation points, or question marks in this sentence unless they are necessary for the context in which the sentence is being used.

4. Example:

Incorrect: My sister is 5 years old!
Correct: My sister is 5 years old.

By following these punctuation rules, you can ensure that your writing is clear, concise, and grammatically correct. Remember to always use a comma between the subject and the age when stating someone's age in a sentence.