A relevant paper is Approaching the Limit: Long-Term Trends in Late and Very Late Fertility (Billari, Kohler, Andersson, Lundstrõm, Population and Development Review, March 2007). Their conclusion indicate that, for the moment at least, late childbearing isn't a very significat demographic factor.
Our empirical analysis of Swedish data shows that since the 1980s the numbers of births to women aged 40+ years and 45+ years have been rising. However, fertility at ages 45+ still constitutes only a minuscule fraction of overall births in 2005, and the contribution of late and very late fertility is currently far below the levels observed in the twentieth century until about 1960.
In effect, there were in 2005 3,420 births to women aged 40 and over, or 3.4 percent of total births, while at the beginning of the previous century they accounted for more than 12 percent. It is important to remark, tough, that low parity births at advanced ages is a relatively new phenomenon; traditionally, children born to mothers aged 40 and over were high parity births.
One of the reasons for the relatively low number of late births is the fact that assisted reproductive technology is still a nascent technology, relatively speaking. Quoting again from the paper,
In 2002, the probability that a 40-year-old childless woman will ever have a child is 7.48 percent, compared to 3.26 percent in 1970.
These numbers indicate both the undeniable technological progress, and its inherent difficulty. This difficulty is one of the factors that make the decision to have children at an advanced age a complex one, even in the absence of (or with lessened) societal and economic constraints. They are still procedures with significant rates of failure, and with important psychological and physiological consequences.
At the same time, this suggests that actual late childbearing must still be below the potential demand for it in the presence of more advanced assisted reproduction technology. As there are significant indications that these technologies will undergo important advances in the future (e.g., via our increased knowledge of the genetics and molecular biology of aging), it is likely that we will see a much larger number of births to mothers of advanced age, above and beyond the increases due to general population aging.
Whether this will have a significant demographic impact or not remains to be seen.