Dynamis can be really difficult no matter what level you are. Depending on your job, you can be useful or useless at any level between 65-75.
For the most part, only WHM, SCH, BRD, COR and BLM seem to be okay starting off at 65. If a shell is using RDM's for sleeps, RDM's need to be 75 and merited. NM's, PLD mobs, WHM mobs, can still be a pain in the rump with 312 Enfeebling Magic, and Sleepga at LV70 is a joke, even with capped Enfeebling. RDM healer though, could start at 65 I suppose, just don't expect much out of them.
Personally in my LS, I recruit mages & COR LV70+ and DD/Tanks @LV75 only.
It seems to me, that the more lower levels you have for Dynamis, the more people you need total to clear. /shrug