Do couples having children later in life act as an evolutionary mechanism?
Many factors have to be considered.  First, although more women are delaying pregnancy until after the age of 35, the overall percentage of women doing so remains very small, as shown by this figure from this New York Times article. Only about 9% of pregnancies in the United States (for 2008) were in women over the age of 35. Most pregnancies still occur in younger women. I suspect this is especially true when considered globally.

Getting pregnant later in life is more difficult, is more likely to result in miscarriage, and is more likely to result in some types of birth defects. The issues are discussed by the Mayo Clinic, WebMD, and summarized by Parenting Magazine. On average, individuals with birth defects are themselves less likely to reproduce (Skjaerven 1999).
For natural selection to favor a particular trait (such as later pregnancy), the individuals with the trait must have higher relative fitness, meaning they produce more offspring on average compared to other individuals. However, relatively few women have late-age pregnancies, more of the offspring are likely to have birth defects, and those offspring are themselves less likely to reproduce. In other words, on average, late-age pregnancies would have reduced fitness compared to individuals that get pregnant at earlier ages.
Most importantly, though, the decision to wait until later in life is a conscious choice, not a genetic trait. Natural selection can only work on genetic variation associated with particular traits. If the choice for delayed pregnancy has no genetic basis, then it is invisible to natural selection, and so cannot evolve.

Citation
Skjaerven, R. et al. 1999. A population-based study of survival and childbearing among female subjects with birth defects and the risk of recurrence in their children. The New England Journal of Medicine 340: 1057-1062.