That's fair. I'd feel pretty uncomfortable on most bikes at 25mph, but I've happily gone 18 without stress on an empty road on a clear day. I think most people who don't have a lot of experience should stick to 10-12 and work their way up, much like learning to pilot anything.

Safe braking distance depends on how much you weigh, the quality of your gear, and the road. I'm 6'4" and weigh 50 more pounds than I did as a scraggly youth when I did faster speeds, so unless I'm on equipment I trust very well I wouldn't often go over 15, I think... for me, most bikes don't brake well enough, and some aren't balanced well for my height.