When combined, 多 (how) + 大 (big) literally creates the question “how big?”. This concept is applied both to physical dimensions and to the “bigness” of one's age.
The most important cultural lesson of “多大 (duō dà)” is its role in asking for someone's age. This is where it contrasts sharply with its counterpart, 几岁 (jǐ suì).
Comparison to Western Culture: In many Western cultures, directly asking an adult their age, especially a woman you don't know well, can be considered rude or overly personal. In China, while it's still a personal question, it's far more common and socially acceptable. Asking someone's age is often a way to establish social context and hierarchy. Knowing if someone is older or younger than you helps determine the appropriate way to address them (e.g., as an older brother `大哥 dàgē` or older sister `姐姐 jiějiě`) and show the proper level of respect, which is a key part of Chinese social interaction.
“多大” is a high-frequency phrase used in a variety of daily situations.
Its connotation is neutral and its formality is suitable for most informal and semi-formal situations. For asking the age of a very respected elder, a more formal phrase like `您多大年纪了？(nín duō dà niánjì le?)` is often preferred.