Could hereditary fructose intolerance be left undiagnosed until adulthood?

Yes: Some forms of the disorder may not manifest until adulthood. "Froesch et al. (1963) described 2 adults, aged 33 and 39 years, with fructose intolerance. In addition to the aversion to fructose-containing foods, both had a remarkable absence of dental caries. Swales and Smith (1966) described an affected 21-year-old man, and Kohlin and Melin (1968) reported adult cases." See http://www.omim.org/