Just because the spot for the months on his age is blank does not mean that Henry Dorges was 18 years and 0 months old when this declaration was signed. He could have not provided his age with more precision than 18. He might have simply guessed at his age. It’s hard to say, but saying that this declaration was made on his birthday is a bit of a stretch. What is safe to say is that Henry indicated he was 18 when he signed the declaration.
Whether that age was correct, accidentally wrong, or an outright lie is another matter.
The year of the declaration is not included in the portion of it used to illustrate this post. There’s the second tip–screenshots and clipped versions of record images sometimes are not enough.