Yellowing older leaves can be normal for a variety of reasons, including natural aging of the oldest leaves (which are almost always the lowest leaves on the stem as the plant grows taller). They turn yellow/brown as the plant lets them die off and then fall off on their own, though you can cut them off if you prefer not to wait until they are fully brown and dried. If older leaves age faster than normal and don't remain on the plant as long, that could indicate that a plant isn't getting enough light, is in need of nitrogen-focused fertilizer, or could use repotting into either a larger container (if roots are too crowded) or the same container with fresh potting mix. Insufficient indoor humidity, exposure to too many warm/cold drafts, and over- or under-watering can also be contributing factors. The plant pictured looks healthy, so it's probably just normal leaf aging or perhaps an indication the plant could use a bit more light this time of year (winter). While correcting any issue like insufficient light won't reverse the yellowing in leaves that have already started to change color, as long as the issue doesn't worsen, the plant is likely fine.

Emily

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