Ages: 18 (Kate) and 16 (Annie)
At ages 14 and 13, sisters Annie and Kate Madden founded FenuHealth in 2015, which produces supplements to prevent stomach ulcers in horses.
FenuHealth began as a winning project for BT Young Scientist, a science and tech exhibition. The sisters got their big break when they attended the world's biggest equestrian trade fair, Equitana, a few months later and introduced their branded feed.
While the Maddens don't disclose turnover or profit, revenue growth in 2018 was around 52%, reported The Irish Times. Product prices range from $50 (€45) to $2,032 (€1,800). And according to BBC News, FenuHealth is used by five royal families.