I study biochemistry at my university. It seems like most of the younger faculty come from a cell biology / molecular biology background, while older professors seem to come from more of a physical science / biochemistry / enzymology background (i.e., chemistry and math meets biology). From other universities in my state, this also seems to be the case. Why is this so?
Yes, this has changed over time, as the number of easily answered open questions in classical biochemistry, enzymology and metabolism is reduced over time. This is much like chemistry itself, where many people (rightly or wrongly), believe that the fundamental principles of chemistry are effectively solved and all that is left is to apply them to real world problems (which many more academic types find less appealing).
This can be seen in the evolution of our own departments, where the department of metabolism (the department where Krebs worked out the TCA cycle) became the department of biochemistry, which was merged into the department of Molecular Biology and Biotechnology, which has just been merged into the School of Biosciences.
There are still groups that practice such things as figuring out reaction mechanisms for biological enzymes, or look at the various physical properties of biological elements, but they are rarely numerous enough these days to sustain a whole department. Instead, these days many people use the principles discovered through classical biochemistry, and the techniques founded by biochemists to student biological problems at a higher level of abstraction - how do the components that biochemists have revealed come together to form organisms and when they go wrong, how does that result in the organismal level phenotype.
Its worth pointing out that a similar process has happened in classical genetics. It happened earlier in biochemistry though. Note that while there is a nature genetics, nature immunology, nature neuroscience etc, there has never been a nature biochemistry.
This is not to say that their arn't still many people and openings working at the interface of physics, math, chemistry and biology. In many ways there are more than ever, but they study different problems - things like genomics, proteomics, gene regulatory system simulation and modelling and structural biology are all highly quantitative. Indeed, many feel the importance quantitative skills in biology in on the increase after a period where it has been left to slide because of the "unreasonable success of molecular biology".
I'm less familiar with those specific fields but I can answer a bit from my own field in biology, neuroscience.
The word neuroscience is quite new, credited to the early 1960s. Neuroscience degrees and departments are even newer; I earned my PhD from one of the earliest graduate programs in neuroscience and it was started in 1971. Even so, there was no "department of neuroscience" until a very recent (I forget the year, approximately last decade) merger of the physiology and anatomy departments.
Of course neuroscience as in the study of the nervous system is not nearly as new, and it's quite straightforward to talk about a history of neuroscience that exists before the word did.
I think you can say much the same for the fields of molecular/cell biology, though I can't give the relevant years. But it's a simple fact that no one doing neuroscience in the 1960s (and there are a ton of key findings in the field in that time period) had a degree in "neuroscience". Most of their students didn't, either, because most often students get a degree named for the department that their advisor is a member of. It takes some time to reorganize academic departments and rename degrees.
Of course as technology advances and key research questions in a field shifts, the number of people doing different types of research also shifts over time. I would not overemphasize the names of certain degrees in peoples' backgrounds, though, and I would caution against extrapolating trends you see in one university to a research field as a whole. These are certainly reflective of some reorganization in the field but it isn't a one-to-one relationship.