I've only just started on my 3rd generation, but so far I haven't seen any particular difference as to how old they are before I separate them. I can't tell how mature the individuals are anyway, they get to a certain size and stay that way for quite a while. I just pull out some of the larger ones and most start to curl within a week or so of being separated. Some will molt first, others just go straight away. I've heard even the smaller ones will do the same though I haven't bothered to try it.
My first experience with mealworm larvaes was with superworms. My husband had a scorpion and a few mealworms we dropped in burrowed into the soil. When his scorpion died we dumped out the tank and found a bunch of giant beetles in the bottom. They were kind of creepy, but cool. I would like to raise them someday. My axolotl loves mealworms although they're not too good for him because of the fat content. He's started spitting out his redworms. I think even animals prefer junk food sometimes