“All children under the age of 13 must be accompanied by an adult.”
You're at a hotel swimming pool. There's a sign hanging near the pool which lists the rules. One of the rules says that children can't swim alone.
All children under the age of 13 must be accompanied by an adult.
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(something is) accompanied by (something)
The word "accompany" means to go somewhere together with. For example, one of the most common uses of this word is in this sentence:
All children must be accompanied by an adult.
You can see this on signs at places that are dangerous for kids, like swimming pools.
It can also be used to talk about documents that need to be sent together:
Please note that all applications must be accompanied by two letters of recommendation.
The word "accompany" is quite formal, so it's mostly used in writing.
(something) must be (done)
This is a form that people use for giving formal instructions. It means that everyone has to do this. For example:
Seat backs and tray tables must be placed in their upright and locked positions.
under the age of (a number)
Usually, when you compare someone's age to a number, you just say "under" or "over" like this:
You're not supposed to drink in the U.S. if you're under 21.
But in more formal writing or speaking, you can use the phrase "under the age of ___":
Persons under the age of 21 are prohibited from buying, consuming, or posessing alcoholic beverages.
An "adult" is a grown person. Usually this means someone who's over about 18 years old, but sometimes the qualification for being an "adult" includes being emotionally mature and responsible.