Definition of thirty-something (forty-something, etc.) in US English:

thirty-something (forty-something, etc.)

phrase

informal
  • An unspecified age between thirty and forty (forty and fifty, etc.)

    ‘I'm guessing she's forty-something’
    as noun ‘she writes a column geared to twenty- and thirty-somethings’
    • ‘Like most of the All-Baseball.com writers, they are also thirty-something.’
    • ‘I was nineteen, going on ten, she was thirty-something and living with a German poet but she liked collecting people.’
    • ‘He was about thirty-something and looked dirty; hadn't shaved in weeks.’
    • ‘It was, she told her fellow researchers, as if a nonagenarian suddenly looked forty-something.’
    • ‘But what is so significant about being thirty-something and being ready to lead?’
    • ‘I had promised myself that I would let loose and live like I was eighteen and not thirty-something.’