I’m not sure one can say that older singles go off the derech, but they do become less enthusiastic about yiddeshkeit in general.
It is very hard to keep up the inspiration from high school and seminary many years later, especially since many girls end up in college or a workplace that is not a frum environment.
I know many older singles, whom when they started dating at 19 they were looking for long time kollel, but as the years went on the requirements dwindled and now they are not interested in kollel at all.