Infant is

Legally an infant is someone under the age of majority. That frequently is 18 for females and 21 for males.

So a legal document refers to a male as an “infant,” he could easily be 21 years old.

We’ve used this tip before, but it bears repeating and remember—in legal documents words are used in their LEGAL context (and based upon specific legal definitions), not necessarily the way we use them in everyday conversation.