He believes it was built in the late, not mid 1600s, still very old.
Yeah, that's about the oldest you can normally find in this country, which is very cool. As noted before, 1695 is the oldest building still standing in Williamsburg, a town that's been keeping records since something like 1635. Our house is very old, but your buddy's place was already onto its second generation by the time the oldest part of ours was built, and probably close to 80 years old by the time the "main" house here was occupied.
I'm not sure how to determine the age of it.
I had a whole mix of various historians and old-house nuts come through this place, when we first bought it. I learned they often disagree, many while being completely cock-sure of their own unsupported conclusions, in the process. But as I learned more about the history of the house, I found many reasons that supported some of their incorrect conclusions. For example, one guy absolutely
insisted the house must be 1820 - 1840, he insisted there was absolutely no way it could be any older than that, based on the construction of interior walls, plaster, etc. Later I learned that the house, while much older, underwent a massive renovation under the generation that owned it during that time window (1820-1840). So, no wonder one historian picked up on that, he just didn't dig deep enough to see the older house hiding behind those updates.
A deed search is always first, which can be quick and easy or slow and painful, depending on age and quality of the records in your area. If anyone notable lived in your house (eg. my pastor), then local church or business records may contain some clues. Finally, track down every prior owner of your house, and get in touch with them or their kin, to get as much info as you can. I had distant-past owners and their grandkids sending me photos, drawings, family trees, histories, etc. As noted, even before finding the date stone, we had the age of the main part narrowed down to a 6 year window. I also believe I know the construction date and owner of the older house upon which this was built atop.
If there are remaining original items, such as door hinges, door hardware, windows, etc., then there are a lot of books that will show you the age of those items. Hinges are nice, because L hinges and H hinges denote specific periods and styles. Of course, your house could be like mine, they replaced most of our 1700's hinges in the 1820's, just the same as you ditched that 1968 Avocado-colored Sunbeam kitchen range 30 years ago, when it was 25 years old. Just be careful to note that things can vary widely by locale.