I've heard the mother's age at conception is important (below 35 usually best) for a baby's health. How much of a factor is the father's age?

Age effects. Women get the blunt of aging effect due to the basics of reproduction.Women are born with all the eggs they will ever release and the effect of background radiation/toxic exposures add up.Men produce sperm from stem cells and essentially make a new batch daily.The healthiest and best swimmers are the most successful for conception.This natural selection spares some of the age issue for men.
Small. Unless the male partner is 50 years of age, or usually even older, the role of paternal age is usually very small compared to that of the mom - not zero, but small.
Pregnancy. Good question, im curious myself. I will have to look it up or talk to colleagues. However, the age of father probably won't have much of an effect on baby health and prenatal-natal-post natal care once you are pregnant. Sperm production decrease with age due lower seminefirous cell function. Health of baby after birth depends on gentic, enviromental factor, and good health habits.