
Caro, I've heard the age mentioned variously as 35, 40 and "any woman over 30 should do some resistance training". My paternal grandmother had very bad osteoperosis and it worries me, however her osteoperosis was really bad due to the fact she slipped on ice in her twenties and shattered her hip, was more or less housebound from then on. A dietician friend of mine told me I was higly unlikely to get osteoperosis due to my lifestyle but a bit of weights would probably help.
One of the reasons I started running was to give my joints a pounding as I was only doing cycling and a bit of climbing for a couple of years. I also have started doing weights again at home. I have done them on and off for about 10 years, I also hate going to the gym so I do free weight mutiple joint exercises at home (squats, deadlifts, chest presses etc) for about 20 minute sessions else I get bored. I also tend to do higher weight low rep stuff for the same reason.
When I do weights consistently I find it helps with bike and (rock) climbing strength and being a girlie I'm not going to pack much muscle on so can get all the benefits without the bulk. Still generally a bit boring though....
I recently started taking Glucosamine on the advice of my Chiro due to my joint hypermobility, its a long term thing but I've noticed my knees don't crack anywhere near as much as they did before (elbows still give stonkers though) so its doing something. I've been taking fish oil for years as I can't stomach oily fish, don't think it does that much for my joints or it may be they'd be a lot worse without them.
If you want some really good and amusing advice on women why they should do weight training have a look at the stumptuous site, it inspired me to dust off the dumbells...
http://www.stumptuous.com/cms/index.php