Can you tell me what age I need to be to buy vitamins and I shop in the uk?

Beyond 100, if then, except vitamin D3 (the only common deficiency in 1st world countries). Vitamins=single molecules common in so many things humans have long eaten humans have not retained every single gene for every single step to manufacture these molecules, generally only missing a single step. All discovered in unusual circumstances for long periods. One known; formed basis for a big industry & sales to naive.