Yellowish leaves base of tomato plants

in Plants
I have two cherry tomato plants growing in my South facing window sill. The lowest leaf on the plant goes yellow. I remove it and another one goes yellow. There are no problems on the leaves higher up the plants. The plants are about 4 1/2 feet and are otherwise healthy.
Anyone know what turns them yellow?
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Old age . ... it comes to us all
...mine are doing exactly the same ... as long as the young leaves are ok you have nothing to worry about.
Ok, thank you