I'm not sure I'm in the "older" crowd, yet, but my toes are on that line, I think LOL I'll be 30 when I have this baby (I'll be 26 weeks tomorrow).
This will probably be very long winded and kinda all about ME because I will try to speak from personal experience rather that quote rules and numbers at you
I'm with the "listen to your body" crowd.
I was running, cycling, and lifting before I got pregnant. I had to give up running very early on because of horrible lower back pain (not pregnancy related), so unfortunately I can't speak with personal experience about that. I do know, however, that it's entirely possible to keep running but at a slower comfortable pace and for shorter distances (I was going to do 2 miles, max). If you're one of those people who can run a marathon every day then you could probably do muuuuch more

When it starts to hurt or strain, slow down and go even shorter. When that gets to be too much, switch to walking.
Cycling....I kept going to spin class (not riding my real bike so much...scared of falling!), but I would wear a heart rate monitor and keep my HR to 185 or below. For me that's 15 bpm lower than the max I usually can hang out at for a while. I would also frequently slow down enough to get my HR to about 130 or 140 then pick up the pace again. With the whole heart rate issue, your PERCEIVED effort is more important than the actual numbers. You'll see different "rules" for the max you should allow yourself all over the place, but as long as you aren't huffing and puffing and getting red in the face odds are you're good. You can still be nice and sweaty by the end, but if YOU aren't getting enough oxygen, neither is baby.
With lifting, I haven't had to lower any weights (not that I could lift all that heavy to begin with, though...), but I've had to modify the positions in which I do them. For the first trimester and a little into the second, I didn't have to make any changes at all. I could still do weighted inclined situps, for example, and work with DBs standing up. Now I only use very light weights with the abs and I don't do anything lying flat on my back. The weight and position of baby and uterus puts pressure on a main artery and can cut off/limit oxygen to both you and baby. But that's something you can start to feel. When it gets uncomfortable or feels just "weird" like it did for me, stop doing things on your back.
Now that I've gotten bigger and my tummy is getting stretched out certain things feel strained. I've started doing things seated that I would normally do standing up with DBs. Just seated stopped being good enough so now I use the adjustable bench so I can rest back on it. Using cables started making my tummy feel strained, so I only use levers/machines that allow me to sit. When that gets to be too much, THEN I'll lower the weight. There have been a few times I've been able to increase weight by just a little (2.5-5lbs. Maybe could have done more, but I'm going to play it safe!), but I wasn't TRYING to. The 'old' weight just got too easy naturally.
Oh what else......that whole thing about not being able to do abs is a bunch of CRAP!! LOL My fitness junky OB nurse got on the floor and specifically showed me side crunches as something you can do "until the day you deliver." I think abwork is something that *REALLY* depends on the individual. If you had super strong abs to begin with you'll be able to do more for longer than the average person. Again, you just have to stop before you feel strained because you can really pull things in that area or end up with abdominal seperation (which can happen anyway, but you can make it worse.). I'll just tell you some of the things I still do:
planks
Hanging leg raises (although you kinda have to bend your knees and turn your legs out a little to make room for the belly eventually hehe)
crunches on exercise ball with a small weight (10# or less), but I don't go back all the way
side crunches w/small weight
obliques on hyperextension bench
That's all I can think of at the moment. My reps are way lower. Maybe I could do more, but yet again I'm playing it safe. I do enough that I feel like I've done SOMETHING, but I don't get sore afterward and I'm not trying to KILL the abs like I used to.
I can't stress enough to pay very close attention to what feels good TO YOU and what doesn't! You've already talked to your doc, so that's good. And just work on maintaining what you can rather than pushing yourself or having lofty goals. Modify, modify, modify
Aaaand I'm tired of "hearing" myself so I'm going to stop now hehe Check out my journal if you wanna see what I get up to

Ask any questions ya want!