Why all young counselors?

Well, I think they stay behind and not choose because they are too young to be resigned to memories. When you are young you look ahead not behind, and if your life were to be cut short too early I imagine it would be hard to adapt to the idea of living eternity with just one memory, and probably one picked out of a much smaller collection than if you had lived longer. Does it make sense? There is the younger charachter that later becomes one of the counsellors, he mentions that he looks at the future, and wanted to make up a movie to hold as his own everlasting memory - but future wasn't acceptable to the law of the place....

They are not all "young" per se -- Mochizuki-san was born in the Taisho reign and died just during the last-ditch phase of WWII in May 1945. Of course dead people are no longer subject to the aging process, hence even though he would be in his late 70s he looks like around 20.

But these caseworkers are all souls stuck in the limbo, and another young guy by the name of Iseya-kun could not choose anything blissful either (neither did Shiori, who died at the age of 18).

So you may be right -- Younger one is, there are less experiences to choose from, and they may be more likely to die unpleasantly than, say, those who died in their 80s and 90s.

Because any older film maker who found themself forced to make low budget movies would become bitter, give up and become a film critic.

I don't think its necessarily because they have so few memories to choose from. In fact, it almost seems like in that aspect, it would make the task easier. However, psychologically, we have tendency to cling to more parts of our egotistical existence when we are younger. You know how when youre in middle school or whatnot, every part of your social life seems new and exciting and important and you want to record every detail? As we age, our brains mature and learn to adapt to the reality of time; that time is precious and we should use our memory for what is most important to us. A study was done taking people of different age groups and showing them a series of photographs. Some of the photos were gruesome and negative while some were happy and heartwarming. At the end of the study, everyone was asked to recall as many of the images as they could. Younger individuals remembered almost all while the older folks consistently recalled only the positive images. Their brains discerned what was most valuable and acted accordingly.