子供が何歳でスマホを買い与えますかって英語でなんて言うの？

Both of these answers are effectively the same as asking "How old will your child be when you give them their first smartphone?". The first answers being a little more formal than the second answer.

"How old will" is another way to ask about someone's age in the future. I hope that this helps :)

As a child gets older, they are a lot more responsible in many different ways. When a child is growing up, parents might give a child a smartphone so they can use it to communicate with them in important situations and emergencies. If you want to ask about the correct age for someone to give their child a smartphone, you may ask some things such as: 1) How old will your child be when you give them their first smartphone? 2) When you buy your child their first smartphone, how old will he/she be? 3) How long are you going to wait before you give your child their first smartphone?

Well, in any reference to someone's child, you should use the child's name (if you know it). When asking a question and you want to know the stage in someone's life that something occurred or will occur, then you can begin the question: "At what age...?"

"At what age would you give your child a smartphone? " Because you are asking 'At what age...' you don't need to really say 'first' smartphone (it is implied).

This question is a little more open-ended than specifically requesting to know about age. For instance, a listener could reply by saying 'Whenever I think they are mature enough.'

The two questions you see provided above are great ways to ask a person at what age they will permit their child to have a smartphone. In the first sentence you will notice the word let. Although this word has different meanings in this sentence it means to allow. This is a word that is appropriate for both formal and informal settings.