In essense, you must be of equal or greater level than your strider to ride without any penalty. The higher the Strider is compared to your level, the greater the penalty.
...soooo, if you want to use it on your subclass, leveling it doesn't sound all that hot...that or delevel it alot. >_<
Isn't there one? Every few levels... I think every 10?
From the info on bravehost, their skills go up every 10 levels up to L70, and every 5 levels after that. Their is no indication that their runspeed changes though. So it appears as if the only other changes are to their HP/MP, patk/pdef and matk/mdef at each level up.
I'd check, but my hatchlings aren't quite up to the level to become striders yet.....only 4 more levels to go, but it's a bit slow since I partner them with my Dwarves for leveling. (Wind x2)