When should a woman have her first child by? For example is there a sharp increase in pregnancy risk over a certain age?

When she is ready. There is not an age when there is a "sharp" increase in pregnancy risks. Generally speaking, pregnancy risks tend to increase with age. That also applies to risks for genetic anomalies (like down syndrome), that risk at age 35 is 1 in 270 and it increases with each year. There is no specific age that is recommended as "best" to have your first child.