I was 43 and my husband was 45, while our birthparents were 19 and 23. We adopted our son at birth (domestic adoption). Our age was not an issue for the agency or the birthparents. It is generally true that agency age requirements are less rigid than in the past, as the incidence of infertility and the age of pre-adoptive parents has risen. Although age may present a slight handicap, it's been my experience and that of many others I know that birthparents are looking for many other factors others than age. At 40 your sister is actually not that old compared to many others seeking to adopt!
Trabot I appreciate your reaction to the age issue - one of the hard things about dealing with infertility and adoption is that not infrequently one's motives and choices are questioned when, had one been able to have children the "old-fashioned" way, no one would dream of raising the same concerns. Of course most people who want to adopt DO give their choices a great deal of thought - for us one of the reasons we are unlikely to attempt to adopt again (at least an infant) is because we are now 47 and 49, and the age and energy gap is more daunting. But that's our decision, and the birthparents', to make.
My advice to your sister is to focus on picking an agency she and her husband feel comfortable with, and to present their information to birthparents (album, dear birthparent letter, whatever is required) in a way that reflects who they are. It's amazing how birthparents can see right to the essentials and not be deflected by age, appearance, financial status, etc. Also, though I am not very familiar with international adoption I believe there are some countries that will allow adoptions of very young children. Some dear friends brought their daughter home from Cambodia when she was 2 1/2 months old!
Tell your sister there really is hope - all of us here, in the myriad of ways we formed our families, are evidence of that!