It apparently relates to people of working age - I'm assuming between 16 and 64, and the kicker or focus is the number of young (20-40) people choosing apparently not to be taxpaying employees. My guess is that they are mostly doing tax-avoiding side-hustles, such as Influencers on TikTok/YT etc, OnlyFans porn, buying and selling stuff on ebay/vinted etc, and drug-dealing (as much of it goes on without Police intervention). I'm sure there are a lot of older people also taking early retirement but there can't be that many given the state of pensions in the last ten years or so, and the taxation of buy-to-lets.