This sentence is equivalent to "What's up" in English and "mat1 je5 si6 乜嘢事" in Cantonese.
Adding a suffix "ér儿" to the word "shì事" is called "érhuà儿化", it is a phonetic phenomenon in Mandarin Chinese. This phenomenon entails the addition of the suffix "ér儿" to a word, merging it with the preceding syllable. It usually does not impact the original word’s meaning.
When you know that a person should be older than 10 years old, you should use "duō dà 多大" (how old) to ask about age. "jǐ suì 几岁" (how old) is not appropriate in this case because "jǐ 几" (how many) is usually used to ask about numbers below 10.
