After how many years are Tefillin supposed to still be acceptable and Kosher? Do people often pass down Tefillin from generation to generation? If so, how old is the oldest pair still in use?
Within the scope of this question, I'll have to in some sense, "redefine" the term "tefillin". You'll see why.
The "foundation" of the tefillin is the parchment on which all the words are written. Technically good quality parchment has an almost indefinite lifespan. Think of the Torah scrolls that you've seen in shuls. Some are a century old or more. Some are in better condition than others. Part of that has to do with the thickness and quality of the parchment, and part has to do with the environment the scrolls are kept in.
The parchment of your tefillin are the same way. You do have one advantage, here, in that the parchment is not exposed to air quite as much as Torah scrolls are, but they can get some dampness if you happen to store them in a damp place. So, to start, when you buy tefillin, try to get some idea of the strength and quality of the parchment. Ideally, if you could find a sofer who is in the process of writing the scrolls that will go into the tefillin housing, that's best, but otherwise, try to get a recommendation to someone who is honest and straightforward. If the sofer doesn't mind opening the housing before you buy it - terrific.
Next, the housing of the tefillin needs to be fairly thick and sturdy. There are different "grades: of thickness, so the thicker the housing is, the better it will protect the parchment from dampness.
The straps get the most exposure, and there are different thicknesses and grades of straps. Regardless of what you do, straps will have to be replaced every 3 - maybe 10 years from what I've seen depending on its thickness and quality. Fortunately, the straps aren't overly expensive compared to the other parts of the tefillin.
The common custom is to have tefillin checked once every 7 years. regardless of the quality of all the parts. The main concern is that letters on the parchment may rub off and the sofer needs to manually correct something. A layman usually doesn't know what to look for so don't try to figure this yourself. The sofer will also check for any dampness / mold / wearing or other problems with the housing and possible problems with the parchment. As I said, I found that rarely does the parchment go completely bad. The housing may have to be replaced and, as before, probably the straps.
The key to all this is proper maintenance and care. If you keep it in a fairly dry place (I don't recommend a typical basement, for example, and not in a humid room during the summer.) and have it inspected on a regular schedule, it should last indefinitely and both you and your progeny will make good use of it for numerous years.
I think I have the tefillin from my father-in-law which is probably close to a century old, and in usable shape. I'm not saying its pretty, necessarily, but it is kosher and usable as of last check a few years ago.
I couldn't venture any guess as to how old the oldest existing tefillin is or who owns it.
"The halachic tradition through the ages has nearly unanimously followed the position of Beit Shammai and Rabbi Shimon ben Gamliel. Rambam (Hilchot Tefillin 2:11) rules that as long as the batim of the tefillin are intact, the tefillin do not need to be checked. He cites the story about Hillel as found in the Yerushalmi presumably as a basis for his ruling. The Smag (Aseh 22, also citing Hillel), Rosh (last halachah in Hilchot Tefillin ) and Tur (OC 39) rule similarly. The Beit Yosef (OC 39:10) cites the Mechilta (in the name of Tosafot [Menachot 42b-43a, s.v. “tefillin”]) and the Yerushalmi, and rules that “tefillin that are known to be kosher never need to be checked” (SA, OC 39:10)."
Some can last for 100s of years. But the rule is, as long as the batim last, they are good.
There are three parts - the scrolls, the houses and the straps:
The scrolls: Basically, there are two approaches - the Shu"A says (as far as I recall) the Prshiyos need to be rechecked once every 7 years, similarly to a Torah scroll, some say (my Rabbi Z"l held this position) once Kosher it never needs to be checked again.
The houses: Need to be checked constantly for their squareness and wholeness and integrity. Also, they have to be "as black as possible".
The straps: also need "constant" monitoring of their integrity and color.