If top growth continues and the plant is healthy overall than it's most likely just due to age, very common.. Old leaves, or leaves that don't produce for the plant (ex. shaded) will commonly yellow (as the plant recovers all mobile nutrients) and eventually drop.. Nothing to be worried about..

Thank you so much for the reply. Yes, top growth is continuing, and overall the plant seems healthy, other than the lower yellow leaves, and I did discover some green worms eating up some of the leaves, which I believe are tomato hornworm. Again, thank you much :)

No thanks necessary my friend.. I do see some insect damage.. Make a point to get out early in the morning (right after sunrise if possible), as that is when you'll find most of the hornworms feeding - look closely, they're tricky lol.. If you happen to find one that looks like the picture below leave it alone, as it has fallen victim to a braconid wasp. The wasp lays its eggs under the skin of the hornworm, and the larvae feed from within.. Eventually they chew through the skin and spin tiny cocoons on its back. They'll soon give way to a new generation of wasps to kill more of our evil hornworm "friends" lol.. The hornworm will die very shortly after the wasps emerge.