Dragons HAVE to killable early on how else would you be able to use the shouts - it's is as intended and I don't really see how they could change it.
Well, it's not like the game depends on your learning any shouts from dragons other than the original. Everything else you need is taught to you by the Greybeards,
Still, I wouldn't mind there being dragons killable by lower level players, as long as they're obviously smaller, weaker dragons, and there will always be some bigger, stronger level 70 or so dragons that are just impossible for lower level characters to fight. They ARE escapable; as long as the low level guys aren't forced to fight the more dangerous dragons, and are able to identify them at distance, then I see no problem with having them.
Preferably the strongest dragons would also inhabit specific areas, so that low level characters are able to avoid them completely if they wish.