I'm 38 and I love a lot of the instructors mentioned in this thread -- indeed, they are the "staples" of my vidiot collection, aside from a wide array of yoga practices. Gilad, Cathe, KCM and Kathy Smith for strength training. Ellen Barrett and Jessica Smith for flowing, low impact, fun cardio. Actually, Jessica Smith for pretty much anything
I dabble in kettlebells and it's a heck of a workout. Erin O'Brien's Strong Body, Ageless Body is an amazing use of 45 mins of time!
I use DVDs from Kari Anderson and Karen Voight less often, but won't part with them. I've been a big fan of low impact since I started working out 15 years ago, mostly because I need to keep my knees happy so I'll actually stay on the fitness bandwagon!