Extruded commercial dog foods contain high levels of carbohydrates. The limited starch digestive capacity of dogs can change with age. The effectiveness of the extrusion (heat-steam pressure) process applied to raw/by-product feedstuffs (different starch sources in terms of starch digestion) may also differ. Therefore, in this study we determined the effects of age and the heat-steam pressure process on the in vitro digestion of different starch sources in dogs. The in vitro digestion was done in faecal inoculums from Labrador Retrievers of different ages (puppy; six months, mature; two years, and geriatric; eight years). The substrates (barley, corn, wheat, rice, oat and potato flours) were studied for in vitro digestion after both extrusion processes (processed; 2.4 bar and 134 degrees C for 14 min) and a non-extrusion (unprocessed). The extrusion process generally increased the in vitro total gas production and true organic matter disappearance (T-OMd) (at 6-48 h) of barley, corn, wheat, rice, and oat flours (P < 0.05). The extrusion process increased T-OMd of potato flour at 6 h (P = 0.005), but did not change at 12-48 h (P > 0.05). The T-OMd at 6-12 h of barley flour by faecal inoculums of >= two-year-old dogs was higher than that of six-month-old dogs. The T-OMd and gas production of starch sources cumulatively increased with incubation time (P < 0.05). The molarities of acetic acid, butyric acid and toal volatile fatty acids in the fermentation fluids of barley, rice and wheat flours increased with the extrusion process or faecal inoculums of two-and eight-year-old dogs (P < 0.05). As a result, the extrusion process positively affected digestion of starch sources for medium to large breed dogs at >= six months of age. We advise that food meant for medium-size breed dogs that are six months and older should be made with more potato, oat and wheat flours rather than other sources.