The Home of Dexter Beef
Here's a very good Dexter breeder's web site from the UK complete with some recipes:
In the site's Background link there's a suggestion that Dexters on a purely grass based diet are best at 3 years of age. Some breeders have reported good success when supplementing with grain finishing at 15 months. Mine have always been supplemented and while not holding to any rigid schedule I've been pleased with the results at around two years of age.
During the days when short legged bulls were being promoted among the beef breeds one of the reasons was earlier maturation. So in that 15 month to 3 year age range, along with diet, it wouldn't surprise me of some differences between short and long legged types of Dexters.