20歳にならないと（なるまでは）お酒は飲んじゃだめだよって英語でなんて言うの？

These are all ways of expression the alcohol restriction.

not allowed to = to be forbidden to do something

Legal age = the age at which a person enters into full adult legal rights and responsibilities

to reach the age of = to turn a certain age.

（１）は「２０歳になるまでは、アルコールを飲んではいけない」という意味になります。
（２）は「２０歳未満の人はアルコールを飲んではいけない」という意味になります。
もし子供が「ビールがほしいな」と言ったようなときは、（１）は個人的に断る言い方、（２）は、一般的に行けないという言い方です。

There are various approaches to this and it depends exactly what age the child is as this may affect the language you use and the way you use it.
Explanation 1 is a logical approach which gives examples of the kinds of drinks you can't drink and also the exact age you may start to sample these beverages.
Explanation 2 is for younger children who may have little conception of when the age of 20 may be reached..or exactly which drinks fall into the prohibited category.
Explanation 3 is designed to put the fear of god into the child so he keeps away from your wonderful bottle of vintage malt whisky!

You can use any of these expressions.

1. You're not allowed to drink until you turn 20.
   If a person is not allowed to do something, it means that they don't have permission to do it.

2. It's illegal for you to drink before the age of 20.
   If something is illegal in a certain country, that means it's against the law.

The above sentences can be used to tell someone that they are below the drinking age. May you please take not of the differences between 20 years old and 20 years of age.

Legal means established by or founded upon law. Illegal is something which goes against established law. Given two sentences will serve you when you are trying to express yourself.

The legal age for drinking here is 20 years old.'
Every country sets an age at which we are legally allowed to drink alcohol. The age is different depending on which country you are in. We can tell someone that they are too young to drink simply by saying 'the legal age here is 20'.

'It is illegal to consume alcohol until you are 20 years old.'
If something is 'illegal', it is not allowed by law. To 'consume' means to eat or drink something. By saying it is illegal to consume alcohol before the age of 20, we are making it clear that they will be breaking the law if they do drink alcohol.

'You're prohibited from drinking until you are 20.'
If something is 'prohibited', it is not allowed. By saying that drinking alcohol before the age of 20 years old is prohibited, we are saying they are not allowed to do it.

A) You are only allowed to drink alcohol when you are 20 years old.
*Allowed - let (someone) have or do something.
Example -"the dissident was allowed to leave the country"

B) No drinking allowed until you're 20 - You are not allowed to drink until you are the age of 20

I hope this helps :-)

In the first sentence, you have used the noun 'minor', which has several meanings, but, in this context, it means 'a person under the age at which he or she legally becomes an adult'. So, you can tell a child that as a minor, he/she can not drink alcohol until he/she legally becomes an adult at the age of 20.

The second and the third sentences give the same advice although slightly differently. The third sentence adds politeness to the advice by starting with the adverb 'please'.

Legal age-the age at which a person is considered competent to manage their own affairs.

Allowed-the phrase implies that one is permitted to do something or let something happen