@PrincessLea: "Declining religiosity" and "relaxed observance." Semantics perhaps, but how I see it, religiosity tends to refer to dedication and devoutness to a religion, which a decline of results in relaxed observance of the practical aspects of the religion. Also, I didn't mean necessarily "no relationship with G-d," nor such that this decline is exhibited via outward appearance. It could be manifest in both, neither, or reflected in an altogether different manner. Because you're right, we have no way of knowing what someone's relationship with Hashem is like. Perhaps s/he found a more authentic way to connect to Him that outwardly looks to people (who shouldn't be judging) like a decrease in yiraas shamyim.
That said, I do think that there is a negative aspect to this decline. It's so easy to spiritually decline when you are not rooted. As an eighteen/twenty-year-old most people are straight off the boat of the influence of formal education. When someone is in a structured Torah environment she is within said sphere of influence. Once she leaves that, it is up to her own strengths and discretion to fight the influences of secular society. Also, many people are pretty sheltered their whole life until they go to (secular) college or take a job outside the frum realm.
Single or married makes that much of a difference in as much as one's spouse is rooted (mostly it's the guy learning for whatever his schedule allows, but even a small amount a day could have a profound impact on a relationship and marriage). If they are not rooted even as a married couple, then I would agree that their marital status doesn't make that much of a difference. Also, I would imagine that it's easier to be focused on the "right things" when you're trying to build the most important relationship of your life, and ensuring that your family have the proper hashkafos. As a single, you have less responsibility and it's easy to convince yourself that what you do doesn't have that much of impact, especially on other people outside yourself.
I think also that the older the single gets, there is this sense of feeling "left out" or frustrated with the system and she might take it out on her Judaism. It is a lot easier to continue to be idealistic and keep all the mitzvos, chumros, etc. how you were taught them when there isn't this sense of pain (however it manifests...could be anger, could be nonchalance, or whatever). It's a nisayon, and just like other nisyonos that make people slip, many singles don't pass this with flying colors.