I'll be travelling to UK/Europe in June this year and I'd like to rent a car. Problem is, I'm only 19. While I'm okay with paying the surcharge for younger drivers, I heard that I must be 25 to rent an auto transmission car, and if I'm under 25 I'd have to rent a manual transmission car. Is that true?
Short answer: no, it's not true.
Details: I just checked on Hertz (french) website, as for an exemple you can rent a Nissan X Trail auto from 19 yo. I guess it'll depend on the car and the company. As auto transmission are often on - a bit - more expensive car you'll have a restriction related to the price of the car more than the auto transmission.
EDIT: furthermore, from 21yo you can rent - on Hertz France - Renault Captur auto, Renault Talisman auto or Renault Grand Scenic auto... so definitely no you are not required to be 25yo to rent auto car.