A person somewhere in their thirties — 30 to 39 — when you do not know or do not care about the exact age. 'The flat is full of thirtysomethings.' It is also used before a noun: 'a thirtysomething teacher', 'thirtysomething couples'. It is friendly, informal English — fine in a magazine or a chat, not in a form or a report, where you would write 'aged 30-39'. The word carries a whole picture with it, not just a number. A 'thirtysomething' is usually someone with a job and a bit of money, maybe a small child, maybe worrying about whether their life turned out how they planned. That flavour comes from an American TV show called 'thirtysomething' in the late 1980s, which was about exactly those people. So when a writer says 'thirtysomethings', they often mean a type, not only an age. The same pattern works with other tens: twentysomething, fortysomething, fiftysomething. Twentysomething and thirtysomething are much the commonest. You may also see it written with a space or a hyphen — 'thirty something', 'thirty-something' — all three appear in print. One more use, quite separate: 'thirty-something' can just mean 'a bit more than thirty' about a number of things: 'thirty-something people came'. Say that one with a hyphen.

三十幾歲的人（三字頭）。台灣口語說「三十幾歲」、「三字頭」；語氣輕鬆，正式文件還是寫「30至39歲」。