Answer: Peace lilies will normally lose a few of the oldest leaves as the plant continues to grow. Based on the description I think the yellowing is probably normal.
The browning edges however are not. They could be related to soluble salt build up in the potting mix, possibly due to the fertilizer. I would suggest you try leaching the soil to flush out the salts. To do that, water thoroughly and let it drain several times in quick succession.
In general, this plant does well with a minimal amount of fertilizer and is very sensitive to salts building up, so you might try using a water soluble form at the lowest rate on the label and only during the spring, summer and fall. If the plant seems to languish, increase the amount slightly.
Browning can also be related to under watering, but if this is the case then you would also see the plant wilting between waterings.
Browning tips and edges can be related to overly dry air, especially during the winter months when the heating system is on so much. You could try setting it on an evaporative humidity tray -- fill a shallow tray with pebbles and add water just to the top of the pebbles, set the plant pot on the pebbles, make sure the bottom of the pot is above the water and not sitting in it; top up the water as needed to replace what is lost through evaporation.
If your plant has grown a lot, you may need to repot it as well.
I hope this helps you trouble shoot.
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