For the best places, yes you need to hit 50 first. The amount you get in sub 50 dungeons is paltry.

The best way to get myth currently, is by joining Brayflox HM in a premade group, and spamming just the first two bosses. Simply kill the first two then leave and go back. This takes at most 12 minutes for 50 mythology.

However the best way for a fresh 50 to do it, while getting gear at the same time, is by doing the Crystal Tower. It rewards 100 mythology, as well as the chance for getting ilvl 80 armour, in about 45 minutes.

There's also the duty roulettes you can do at 50. Some of them offer a nice chunk of myth, once a day. But again, I recommend doing CT first, to make doing the other dungeons/trials easier for you (by getting ilvl 80 gear). You also may get grief from some douchbag players if you go into something "undergeared."