The Rashba (Rabbi Shlomo Ben Aderet of Barcelona, 1235-1310), in one of his responsa, discusses the concept of "Minhag Nashim Zekenot," or "customs of elderly women." There are certain customs taught by older Jewish woman, which do not, on the surface, appear to have a valid source or reason. The Rashba writes that these customs should not be automatically rejected or dismissed, as we may presume that these are ancient practices that were established for valid reasons.
The Be’er Moshe (Rav Moshe Stern of Debrezyn) addresses a number of these kinds of customs that have become prevalent among certain groups of Jews. Some people have the custom to ensure not to walk over a baby or child, and to require somebody who did walk over a baby or child to walk over him again, backward. The Be’er Moshe writes that this is, indeed, a legitimate practice that is based upon a valid reason. There is a concern that walking over a child will have the effect of stunting his growth, and therefore if one did walk over a child, he should walk back over him to eliminate the effect. (Accordingly, there would be no problem with walking over a person who has already grown to his or her full height.)
The Be’er Moshe also discusses the custom to tie a red string to a baby’s stroller or wrist to protect him or her from the Ayin Ha’ra (evil eye). He writes that this practice, too, is a legitimate custom that has its origins in authentic sources, and therefore it is acceptable to follow this practice.
This discussion demonstrates the fact that not all practices that have been adopted are necessarily legitimate. Customs that people observe as religious practices must be first determined valid based upon Halacha, and should not be automatically assumed to be legitimate religious customs.
Summary: There are many "old women customs" that some people observe as religious practices. On the one hand, these customs should not be automatically discounted even though one does not know their underlying reason. At the same time, however, they should not be assumed to be legitimate without consulting with a Halachic expert.
