What's the difference between 5か月 and 5ヶ月 or they are the same? When to use them? I want to say my dog is 5 Mths old. Thanks!!!!
In your case, か and ケ have the same meaning and can be used interchangeably. As ケ appears somewhat old-fashioned, it may look natural if you use か to write about your dog.
Please note that they are not identical and there are cases where those two have different functions, e.g. you can describe 5 pieces as 5ケ (in this case you pronounce ケ as "ko" in stead of "ka"!), but not as 5か. Expressions related to numbers are so unique in every language, aren't they?