Have a look at the naturalization data for the sons as well. The 1900 census can be a guide to this. If you see that the sons all state their naturalization was about the same year, it *may* mean they didn't apply on their own, but rather they were covered under their father's application. This can help determine the time frame he applied. Wives and minor children were automatically naturalized via the husband/fathers application (prior to 1925).