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?

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.

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.