It's not going to happen. Your ppm in your rez is going to be the same going to every bucket and your only option would be to setup a second rez that was more concentrated and setup a top feed on a timer to feed the older ones but that will just end up back in your rez and you'll be constantly adding more water to maintain your intended ppm for the younger plants.
If you want a staggered harvest, my suggestion is to search diy aeroponics and go that route.
Good luck to you.