Who is obligated?
Men and Women
Chazal tell us that this mitzvah applies equally to men and women. One reason women are obligated is simply because mezuzah is not a timebound mitzvah from which women are exempt. A second reason mentioned is that inasmuch as the mitzvah of mezuzah increases the days of one’s life, certainly women need life no less than men.
There is no question that essentially, children need mezuzos as well. However, children are technically not obligated in any mitzvah. Therefore, adult family members should ensure that the children’s rooms or home have mezuzos as well.
Mitzvah every second
It is clear from the poskim that the mitzvah of mezuzah is fulfilled or neglected every moment that one has or does not have a mezuzah affixed on one’s doorpost. In other words, while some mitzvos are fulfilled once a day, others once a week and others once a year etc. the mitzvah of mezuzah is fulfilled every moment.
I myself witnessed an incredible expression of yiras shamayim relative to this halacha. A particular Rav contacted me and said that a mezuzah in his home had become pasul and he needed a new one. I offered to come to his home with a choice of two or three options that he would be able to purchase as a replacement. When I came to his home, he looked at the various options, we discussed the pros and cons of each one, and he then chose a particular mezuzah and paid me for it. Throughout the process he was warm, kind, and relaxed.
Once he chose which mezuzah he wanted, I sat down, rolled it up and handed it to him. The second I handed it to him, his entire demeanor underwent a complete transformation. He jumped out of his chair, raced to the doorway and affixed the mezuzah.
Noticing my look of surprise he explained, “so long as I didn’t have a mezuzah to affix, I couldn’t possibly fulfill my obligation. However, once I had a kosher mezuzah in my possession, every second I don’t affix it would be neglecting a Torah obligation!”
To be in a home with no mezuzah?
This leads to an interesting question. Suppose one is unable to obtain mezuzos for his home, is he permitted to live in the home? The poskim actually discuss this question at some length, and many posit that although one neglects the obligation to have a mezuzah on his doorposts every moment, nonetheless there is no prohibition to dwell in a home without a mezuzah. However, it must be reiterated that this applies solely in a case where one is truly unable to obtain a mezuzah. If one is able to obtain one with some effort, that effort must be expended before dwelling in the home.
Similarly, if one’s mezuzah fell off the doorpost on Shabbos, inasmuch as the mezuzah may not be reaffixed on Shabbos, must he leave his home until Motzoai Shabbos when he can reaffix the mezuzah? Indeed, some poskim say that if one has another home nearby that he can go to, he should do so. Yet since others say this is not necessary, coupled with the fact that it is often impractical, the custom seems to be in accordance with the lenient opinions.
However, it should be noted that if the mezuzah fell off the doorway of a room in the home, it often is not especially difficult to avoid entering that room until after Shabbos. In such an instance, it seems as though it would be meritorious to be stringent.