Syntax for placeholder text

Sometimes we need to write a sentence with a placeholder for a missing bit of information, which the reader needs to fill out when reading the text. This may be a number, or a word, or a sequence of words, or something else. It may be because the author doesn’t know the information, or he wants to focus on the meaning of the sentence instead of the missing information.

For numbers, we often just replace them by a symbol of a common mathematical variable, like X, or n. Sometimes this may also work with words. For example

I am X years old.
The name of my dog is X.

But often the variable itself doesn’t clearly convey what it should be replaced with. It needs a placeholder text which describes what the variable stands for. This placeholder text is often surrounded by different kinds of brackets, like square brackets [] or angle brackets <>. But it often seems like the arbitrary choice of the author instead of a commonly agreed upon syntax. For example

I am [age] years old.
The name of my dog is <dog name>.

I’m wondering, is there any agreement on what syntax should be used for placeholder text? Also, does there exist different syntax than the brackets in my example?

The bracket syntaxes are both problematic since they interfer with the most common markup languages used. In HTML the angle brackets interfere with the element tags. In markdown the square brackets can get confused with links that miss a URL.