My great grandma died at the age of 94. My grandma said last year in her late 70s "I wish my mom was alive, there are so many things I want to ask her" then started to name the things she wanted to ask her which included "Why didn't she smile in her wedding photo," a photo which has been on the mantle for at least the last 30 years.
If you live close to your parents, do it in person the next time you see them (and bring a voice recorder, your grand-kids will thank you some day). If you live far away, I built a service that makes it easy to do over email (http://www.storyworth.com).