Is it permissible for someone who is below average height to affix a mezuzah lower than the norm, in order that they may be able to touch it and "interact" with it as they enter and exit their home, so that it may not be merely a "decorative" element?
First, regarding the height at which a mezuzah should properly be affixed, the halacha is clear on this matter, and all authorities are in agreement. The mezuzah should be affixed in the upper third of the doorpost, but not less than one handsbreath from the lintel. (See, for example, Ganzfreid's Code of Jewish Law). An average exterior door is typically between six feet six inches (78 inches) and seven feet (84 inches) in height. Thus a mezuzah may be hung 52 - 56 inches high (4'4" - 4'8", depending on the height of the door). A typical person's reach is at least twenty percent above her height. Thus, if the door is seven feet tall, the mezuzah may be hung at a height of four feet eight inches, and should be easily accessible to a person under four feet tall. So, unless someone has a truly unusual door, or they are exceptionally short in stature, they should have no trouble at all complying with the letter of the halacha.
Second, regarding the usage of the mezuzah. Maimonides (Code, Book 2, Laws of Mezzuzah, Chapter 6:13) exhorts readers to be scrupulous in observing the mitzvah of having a mezuzah since "whenever one enters or leaves a home with a mezuzah on the doorpost, he will see it and be confronted with the declaration of God's unity.....This thought will immeditely restore him to his right senses and he will walk in the paths of righteousness." According to Maimonides, the essence of the mitzvah of mezuzah is to see it -- it is not necessary to touch it. Further, Maimonides explicitly warns against those who misuse the mezuzah, presuming it to be an amulet (loc. cit. 5:4). In other words, even if it were not possible for someone to hang their mezuzah within reach, they can fulfill both the letter and the spirit of the halacha by hanging it within sight.
Third, regarding the touching of the mezuzah. The custom appears to have originated with the MaHaRil (Rabbi Jacob ben Moses Boellin, 1360-1427), who based it on a story in the Talmud (Avodah Zarah 11a) in which Onkelos b. Kalonymous the Proselyte touched his mezuzuah in order to be afforded protection against Roman soldiers who were arresting him. So there is some rabbinic precedent to the custom.
The desire to observe this custom, and thereby to mark the separation between the private, sanctified space within and the public, ordinary space outside, appears to be a good example of a positive application of the Reform Jewish principle of seeking renewed meaning to traditional customs and observances. On this ground, I might be inclined to support it, even in the unlikely circumstance that the door is so high, or the people so short, that they cannot possibly reach the mezuzah within the rather broad halachic guidelines above. However, even in that instance, one must consider the principle of Marit Ayin (appearance to others) -- and be concerned that acquaintances, possessed of less intense Jewish identity and literacy, might see the lower-than-usual mezuzah and conclude that that is the normal, appropriate height at which to hang it. This might then mislead their own practice, even if there were no need to do so.
In conclusion -- depending on the height of the door, it seems likely that the mezuzah can be hung at a height which fulfills the halacha and can still be within reach. If not, the mitzvah of mezuzah can still be fulfilled if it is within sight, even if not within reach. I would counsel against hanging it lower, since:
- to do so is not halachically permissible;
- to do so is not necessary, either to fulfill the letter or the spirit of the halacha, even if your friends are exceedingly short;
- the custom of touching the mezuzah is of relatively late origin, is a custom (not a law) and might be construed to lean towards making the mezuzah an amulet; and
- hanging the mezuzah too low might give an erroneous impression to others, who have not researched the question carefully.