I'm a go by the book type person, though.
Most of the food manufacturers (the ones that make separate puppy formulas) suggest changing to the adult version of their product by six months - and usually don't give feeding guidelines beyond that. Plus, their foods, if trialled at all, are only trialled and rated for puppies under six months. Going 'by the book' in that case, would seem to mean swapping to adult food at six months of age
If you happen to put any faith in feeding trials, then all-life-stages foods are probably the best option. They go through more than twice the testing that limited foods do (which still only means a matter of weeks).
Personally, I don't think it matters a whole lot what's on the label - it's what's inside the bag that counts. Some puppy formulas are grossly overloaded - some others differ from the adult food solely by packaging. Same goes for some of the "breed specific" foods, or size specific ones. Best to just read the labelling on the food you're feeding and see how it compares to the nutritional needs of a growing pup. If it's perfectly adequate, then I'd go right ahead with feeding both the same food. And if it's not, then you could probably find something better for the adult dog also.
