「あなたの国では平均何歳で結婚をするのですか？」って英語でなんて言うの？

The two questions you see provided above are great ways to ask someone at what age people get married. In the first sentence you will notice the word normally and in the second sentence you will notice the word usually. Both of these words mean mainly or in general. Both of these words are appropriate for both formal and informal settings. They would make great additions to your vocabulary

This is a long but clear way to ask when people normally get married in their country. When you want to ask a specific question, it's acceptable to use the phrase "At what." I hope that this helps. :)

You don't actually need the phrase 'on average' since the first question implies it. Both of these phrases would suffice.

When talking about an average it is worked out on a group of people so not everyone will do that at the same time or age Every country has different thoughts ideas and cultures so you might want to know about that countries average

In your opinion, at what age do people in your country typically get married? "In your opinion" means that it is only what you think, and not necessarily the exact fact. So in this question, you are asking the person what they personally think. Typically means usually; in most cases. So by asking this, you are asking the person at what age they personally think people usually get married in their country. What is the average age that people in your country get married? This question is asking the person at what age people in their country normally get married or what they think the appropriate age is for people in their country to get married.