The beetles will start laying eggs every day. This will naturally start to stagger the age of the mealworms. If you want to keep them all at different stages, get a bunch of containers and put some wheat bran in each. Then every couple weeks (or whatever) move the beetles into a new container. Then you will know the age of the worms in each bin.
Also, if you want to slow the growth, you can put ones you plan on feeding to the birds in the refrigerator. They don't always do so great once they come out of the refrigerator which is why I say to just put feeders in there. Also, if you have them in a cool room, they will grow a lot slower. So maybe some are in warmer rooms to cycle faster while other are in cooler ones.
You can mix and match any of these strategies. But like I said, they will naturally end up staggered in age once the 2nd and 3rd generation get going even if you do nothing. Patience is the key.
edit: I just noticed that you said they hatched then became beetles fast. I missed that part. So just save some in the refrigerator for feeding out is what you should probably do.