How did I become a grad student at 40, and how has it worked out? I think I was pretty typical for women of the 1950s, in that I married when I finished college in 1957 and only worked until the children came along. My background, however, was definitely scientific. In fact, both of my parents were scientists, and I majored in biology in college (after trying all sorts of other things, as my other major interest has always been art). My husband was also a scientist and I worked for 3 years as a lab technician before having children. So I rather think of myself at that period as a "science groupie," and as a faculty wife, I was exposed to some pretty exciting science during those years when molecular genetics was just developing.
During the time my two boys were young, I did lots of volunteer work, pottery, and other artwork for the next 15 years, and I worked a couple of summers as a course technical assistant at Cold Spring Harbor for my former boss. It was enough to know that I probably wanted to go back into science eventually. In 1974, I worked as a lab volunteer while my husband was on sabbatical in England for a year, and then we divorced shortly after. So (again like a lot of '50s women!) I came to the point of trying to figure out how to both earn a living and do something I liked for the rest of my life.
At that point, I thought seriously about both art and science, but I decided that art was too uncertain, financially, and too solitary, and I wasn't going to use my head in all the ways I wanted to be engaged ... so it was science. The choices seemed to be working as a technician or going to grad school and then, I wasn't sure what, for a long-term job. However, when I'd been a tech, I'd always been given my own projects and was treated like a grad student, which made it fun, and I did think I could have easily gotten into grad school right after I had finished college. I wasn't so sure about 18 years later! So I went both directions at once--I applied to grad school and followed up contacts for a technician's job--and was really quite surprised when acceptances arrived from three of the four schools I applied to, along with job offers! I decided maybe I could make it, and would go to graduate school as long as I was enjoying the process (not expecting continuous fun, of course; grad school always has its low points). Friends and family were really supportive at that time, which was a big help in making this hard decision. In fact, several friends said they thought I should have gone to grad school long before (but they had never said it then, or I might have thought about it!) I did think maybe I was doing something crazy when two of my former undergraduate classmates won the Nobel prize that year, just as I was taking the GREs!
So off I went, rather nervously, to the University of Colorado  in 1975, planning to work for a PI in the same field that I'd worked in during the year in Cambridge ( C. elegans). It turned out to be a very good move, in the long run, but that first year was rather difficult! Being older meant that I had a lot of catching up to do: As one member of my committee said, "I don't think any of her undergraduate courses are relevant." But I'd had a good deal of lab experience, which put me ahead of some of the younger grad students. And I got a lot of support from the other students ... including one who, when introducing himself, said, "My mother told me to take care of you." It turned out I knew his parents, although I'd never met him, and his mother had also gone back to science after raising a bunch of kids.
I don't think I was treated differently with respect to any of the formal requirements, but there were some differences I found as an older grad student. Being older than my Ph.D. advisor certainly made for a different relationship than the younger students had with their advisors. This, in addition to having met almost everybody in my field (which was very new then) during the year in England, gave me some entrees that other students didn't have. People weren't quite sure what to make of me at first, I think.
Although there were no formal support networks that far back, I did get some helpful faculty support. And this being the early years of women's movements, the women from the year ahead of me had organized a group that met regularly as a journal club, called the "Ladies' Home Journal Club." They took in the women of my year (we got rather envious remarks from some of the men), and it functioned as a great support group for dealing with issues of women in science. The only other female grad student in my lab also became a particularly good mentor to me. And, overall in that department, the students and the postdocs socialized a lot together as well as interacting in the course of their work. Upon reflection, I realize that I had many new friends and absorbing work relationships that really helped me through a difficult period in my life. Age seemed to make very little difference--in fact, although my fellow students knew I had children, they did a real double-take when the boys turned up and were taller than I was. (Then they started including them on hikes and inviting them to parties.)
Probably the hardest thing was leaving the boys behind in California, as they wanted to finish high school where they were, so they stayed with their father. In the end, it worked out well, as they each finished a year early and came to Boulder for a year before going to college, then returned for the summers. In my first year, I was able to work hard without the distraction of being a mother, and then in my second and third years, I had one at a time, and really got to know them without the sibling rivalries! And yes, the stipend was pretty small, but I did get some additional support so I could have an apartment large enough for the boys. One year all three of us took out student loans at once!
It took 6 years to get my Ph.D., which is a bit more than I had figured, but I believe I was the second in my class to finish. I did a 5-year postdoc in Canada, decided I didn't want to look for a teaching post at age 50, and in the end came back to the Department of Molecular, Cellular, and Developmental Biology  at Boulder as a senior research associate in the lab of a colleague in the worm field. These days, a biotech job would be another option for the late starter. I've been here 12 years now, still working on worms; I may have to retire to get back to the art one of these days, but the science is still fun and rewarding! It's been very interesting watching the younger women who've gone straight through graduate school, a postdoc, a job search, and then have to deal with the stress of a new faculty job and finally having children. Interrupting first and then going back to school may give fewer career options, but my general take is that it's probably the less stressful option overall.