B.,
I have a 3 y/o and a 1 y/o, and my 3 y/o son has been in a twin bed since he was 2. We had double rails/guards on the bed at first and then after a few months we realized that once he goes to bed he doesn't really move. So early summer we took them off and he has never fallen out of the bed.
As far as "when", I think it depends on how your child sleeps. If they have fallen out of the bed with them on and move around a lot when they sleep, I would tell you to leave them on. If you are tired of seeing them and want them to begin to transition away from them, you could explain it to them and then take them off and put pillows on the floor so if they do roll out, they will hit the pillows instead of the hard floor. (of course make sure there is nothing else around they could hit..tables, bookshelves....).
Another thought is that if you can get your older one to sleep without it, maybe the younger one will follow their lead so you can get rid of both of them, again if they are ready.
Good luck...hope this helps.
A. D