Yellowing leaves
 
Most plant care advice would have you believe that every yellowing leaf is a dire signal that the plant needs some immediate action.  The truth is, older leaves must turn yellow and die off in order for new ones to grow.  What is actually happening: chlorophyll is being broken down and its components are being transported back into the roots - a way of nutrient recycling.  When the green chlorophyll is removed, what remains is reddish-orange caretenoids and yellow pigmented xanthophylls.  So the yellow color of a dying leaf isn't the color of decay but rather, the color of the leftover leaf parts.

Remember that plants adapt to their environment - the number of leaves they can support is based mostly on the long-term average light intensity, water intake, and trace nutrients.  When you do find some older (lower) leaves yellowing after a few weeks of buying a plant, here's a more helpful way to assess whether you need to be worried - consider these things IN THIS ORDER!

1) Light - think about the drastic reduction in light intensity and duration that your plant is experiencing compared to what it received at the nursery.  Depending on the plant, your plant is simply stabilizing its metabolism with the available light - like cutting back on employees if overall revenue has decreased.

2) Watering - what is your watering algorithm?  Is your cue to water a particular level of soil dryness or is it done strictly by a schedule?  The right answer is 'according to soil dryness'.  

3) Soil Aeration - the roots need more than just moisture.  They exchange gases and will die if they are suffocated by compacted soil or overly moist soil (in combination with low light).  Soil aeration is done by gently poking the soil with a chopstick before watering.  This creates channels where air can reach the roots and, as a bonus, it helps the water more evenly penetrate.

If you are doing the best you can with the above three factors, then I would say, for that leaf, its time has come.  Do NOT be under the impression that "perfect care" = "perfect plants".   If you had had brighter light, watered at the right time, and aerated more often, then the leaf would have yellowed a bit later...it's inevitable!  And the plants at nurseries are carefully groomed daily to preserve that image of perfection - it sells!

I've documented the growth of my monstera here.