It's interesting to note reasons for the increase in age of conception. Whilst economic reasons are obvious - waiting to have reached a position of financial stability - I think there are other factors to consider.
In the past 20 years, the number of young people continuing in full time education until their early 20s has massively increased. Obviously, this delays the financial independence needed for child-rearing and further delays the option of having even a home to raise those children in, but it has also had another effect.
When it was much more common for young adults to start working at 15/16, not only were they in a better position financially to have children in their early 20s, it could be argued they were emotionally more mature, having already been working for at least 4 years and often already living outside the family home.
Whilst much is made of the science behind the brain not being fully developed until around 27, we have no reason to assume this has not always been the case, yet far more people below this age in the past seemed capable of living adult lives. It seems that increased levels of being cocooned in the Education system for longer and by necessity being in the family home longer may also be delaying maturity.
Our bodies still operate on optimal fertility levels before the age of 35, but fewer and fewer of us have reached the point of not only financial stability, but the emotional maturity that our predecessors would have developed by this time through necessity. Our children are 'children' for longer than they have ever been. Even allowing for the multi-generational living of the past, there is a wealth of difference between my father leaving school at 14 to work to bring money into the family home and Joe Bloggs bringing his washing home from uni at 20.
Biologically, we still need to be having children in our 20s. Financially and, increasingly, emotionally, we simply aren't ready at that age, to the point where having children in your 20s is beginning to be viewed as almost the equivalent of being what used to be called a 'gymslip mum'.