"Grow up" has 4 definitions and is inseparable.
Because all of the definitions relate to growth, they are also associated with time. For this reason, I will also explain which tenses can be used as I go through each definition.
X Present simple tense
The only tense that cannot be used with all the definitions is the present simple tense. This is because this tense is often used to talk about habitual actions and continual occurrences, whereas the time associated with "grow up" can only take place during a certain period of time.
"He has grown up."
Not - "He grows up."
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Definitions
1. The change or development from when one is young to when they are older.
You can use this to talk about the transitions from a baby or a young child TO an older child or an adult.
Past perfect:
"The last time I saw her, she had grown up a lot faster than I expected."
Present continuous:
"She is really growing up!"
Future:
"She's going to grow up really quickly."
We can also talk about the result of the development, often with the verb "to become."
Past simple:
"She grew up and became a teacher."
Present perfect:
"He's grown up to become a very ambitious young man."
Future:
"She wants to become a pilot when she grows up."
X Past & Present Continuous tenses
When we talk about the result of development, we cannot use the past and present continuous,
Past simple:
"She grew up and became a teacher."
Not - "She was growing up and became a teacher."
Not - "She is growing up to become a teacher."
2. What happened during one's development - specifically referring to the period of time when one is young.
Usually this is used in the past tense, although the future is possible.
Past simple:
"I grew up in Brunei."
Past continuous:
"When I was growing up in Brunei, I went to an international school."
Future simple:
"My nephew will grow up in Malaysia."
X Present tenses
The exception here are the present tenses because this definition in particular is usually used by someone who is older and talking about the past.
3. To stop behaving like a child or become more mature and wiser.
With this definition, a variety of tenses can be used although "grow up" is sometimes used as a gerund, infinitive, or an imperative to tell someone to stop behaving like a child.
Past simple:
"He finally grew up when she got really sick."
Gerund:
"He only started growing up because he lost everything."
Infinitive:
"She had to grow up quickly because her father passed away."
Imperative:
"Grow up!"
4. The start and development of towns or cities.
Although this use is less common, grammar rules related to describing development, and referring to what happened during the period of development also applies.
- Transition from a young town/city TO an older town/city.
Present perfect:
"The city has grown up to become the cultural capital of the country."
- What happened during its period of development.
Past simple:
"The town grew up and flourished by the river."
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