My wife Mary had to wait until she was 23 (read sufficiently ready or prepared) before she could go to the temple to receive her endowments. Someone else I know received her endowments when she was still a teenager; not because she was ready, but because she was getting married two hours later. In fact, I would venture to say she was not at all prepared for the endowment experience. While it would only be guessing, the endowment experience may be the very reason she hasn’t been back for several years and still does not have a recommend.
Anyhow, the point is: why is spiritual preparation important for some, but not important for others? Why is spiritual preparation not a requirement of new brides?