I started just after my 36th birthday and am pushing 39 now. I say go for it. You don't really even need to be in shape. BJJ x3 per week will get you into shape. I trained as often as I could, listened to my body and set goals that were realistic for me. I dropped 40 lbs in the first 6 months of training and feel much better now than I did at 25.
I'll be a little more cautious than others, though. Being 40 or 50 or however old isn't a reason to stop doing things you want to do, but do it smart. At 38, I'm not as resilient as I once was. I'd recommend that your friends get a check up and talk to their doctors about training. BJJ is an intense workout.
They should also listen to their bodies. The kids that train 6 days each week are to be commended, but I just can't do it. I'm good for 4 classes per week plus some open mat and light rolling outside of class. I get too beat up to take care of my family if I overtrain.
I would definitely suggest supplementing their diet with fish oil. The best thing my doctor recommended for me was to superdose on fish oil. He recommended between 8 and 10 grams per day. More than glucosamine/chondroiton or any other supplement, fish oil has been like WD-40 for my joints, particularly my gimpy hip. It's good for your heart, too.
Finally, if they do it, I'd recommend taht they just commit to 6 months. Depending upon their conditioning, it's going to take a while for them to even get their heads above water. I spent the first 3 months just struggling to make it through class. I'm confident that many of the guys at my school doubted I'd last... I was that guy who comes in, sweats like crazy, looks like he's about to keel over and often pukes in the bathroom after a 5 minute round. I was that guy.
Fortunately, in spite of the nausea induced haze, I was having so much fun I just had to go back. Eventually, my conditioning got better... still questionable, but better, and I was able to engage my brain enough to learn some jiu-jitsu.
I'm really not overstating the situation or just saying this. Barring any medical reason to the contrary, if I can do it, anyone can.