How fast are fast mounts shared with lower level characters
Solution 1:
All mounts now will auto-scale to the skill of the rider. If you have a higher riding (or flying) skill, the mount will fly faster, if you are level 20, it will ride at 60% (the standard level 20 speed), regardless of the mount.
There are exceptions, such as the Sea Turtle, which do not increase speed with riding skill.