Ok first of all why would you give a young plant 1/2 strength nutes! not smart at all. i wouldnt start with any nutes until at least 3 weeks to be on the safe side. flush and you may be alright if it isnt too late already.
Plants are never perfect, this isnt that big of a deal. if you want to devote all your time and energy into this im sure you could find an explanation but to be honest, i think that would be a wast of time. your plants look green and healthy. if it gets worse then it would be worth looking into...
Ok, this is not a big deal at all man. when plants mature then usually kill off the bottom old couple of leaves to focus its energy on the rest of the new foliage. "Serious Problem" i think not. i'd chill out and wait and see if the problem progresses, but i think your ok for now.
Alright, Firstly, your plants aren't being underfed because they are so young that shouldn't need any nutes at this point. Secondly, I would transplant those suckers just to be safe, Because a lot of the time a new pot and fresh soil can go along way.
Cheers
If your in the flowering stage, i wouldnt worry to much about it. Its Normal for bottom leaves to turn yellow and died off in flowering. If your really worried about it, i would give it just a bit of Nitrogen to fix up the problem.