Have you had the plant the entire 15 years, or was it acquired more recently? Do you know when it was last repotted into fresh potting mix? Over time, potting mix degrades and can compress and become more powdery, which can lead to issues like less oxygen available for roots, retaining moisture for too long, and being harder to re-moisten if it gets too dry. Old potting mix also can have a buildup of minerals from both the water source as well as any unused fertilizer (if you got the plant from someone else, maybe they fertilized a lot even if you have not), and those mineral excesses can harm roots if not removed by repotting into fresh mix.

Lots of factors can cause leaf yellowing on indoor plants. Over-watering, under-watering, a need for fertilizer to replenish depleted nutrients, hot/cold drafts, insufficient humidity, and not enough light. It's also normal for older, lower leaves to be shed by the plant as it ages, especially for large-maturing houseplants like Dracaena that is actually a tree where it grows wild outside.

From the photo, it looks like the plant isn't getting enough light. A position to the side of a window isn't very bright, and even plants with a good tolerance for low light do best when grown close to a window (or grow lights). A window facing north or east that doesn't get much direct sun in summer is fine to set the plant directly in front of, but in a sunnier window, the plant could be placed a couple or so feet away from the glass (in front of the window, though, not a couple feet to the side, which is much darker for it). In winter, even a sunny window should be fine, since the light intensity is much less that time of year.

Nitrogen is the nutrient plants use in the greatest amount, and some forms of nitrogen leach out of the soil with each watering. When roots don't have enough nitrogen, the plant can take it out of its oldest leaves in order to use the nutrient in younger growth. That results in lower leaf yellowing, since the lowermost leaves are usually the oldest leaves on the plant. While adding nitrogen from fertilizer won't turn any leaves that have begun to yellow green again, it will slow or stop that from happening to more foliage. The fertilizer you choose doesn't have to contain only nitrogen (most will have several nutrients), but that is a good focus since it's the nutrient houseplants (and any plant grown in a container instead of the ground) would need supplemented the most. Fertilizer nutrient analysis list the N-P-K on the package label; N is the nitrogen content as a percentage of the weight of the fertilizer. If you fertilize, you don't need to find something with tons of N, but any general-purpose houseplant fertilizer formulation should be fine. Follow product label directions for use, as each formulation may differ in dosage and how often it's applied.

Dracaena prefers to dry out a bit between waterings. Following a set schedule for watering risks giving the plant water when it's not needed (which not only removes some nutrients from the soil, but also risks the development of root rot) and risks underwatering the plant when more moisture is needed. The best approach is to feel the soil at least an inch or two (deeper for very large pots) beneath the surface. If it is fairly dry to the touch at that depth, watering might benefit the plant; if it's damp when checked instead, then watering can wait. If the plant sits on a saucer to catch drained water, empty it of any collected water once you're done watering so the pot doesn't sit in the water and absorb it through the bottom drain holes, which can also risk root rot. If the plant has been in this pot for several years, root growth might be taking up more space, meaning that the container may be drying out faster since there's less soil volume to retain moisture. Plant moisture needs also vary from season to season (even indoors) and as they mature, so the same watering schedule won't be needed throughout its lifespan, which is another reason to check the soil first instead of automatically watering at a set time.

For now, you may want to shift the plant into the path of more light, and adjust your watering frequency. If it's been at least a couple years since it was last repotted, it would help to remove what old potting mix you can and put the plant into fresh mix. (Using the same pot is fine as long as the root volume hasn't filled the pot too much.) Make sure the pot does have at least one drain hole, though; if it doesn't, that's a good time to move the plant into a pot that does have drainage. Different potting mixes dry out at different rates based on their varied ingredients, so you may have to get used to a new drying pace/pattern of a repotted plant. Since it sounds like the plant hasn't been fertilized in a while, you also could apply a houseplant fertilizer (liquid or granular/pelletized, it's your choice). Be aware that, if you do repot it, some potting soil mixes contain some fertilizer to get plants started on growth, in which case you can skip adding your own fertilizer until next spring. (Houseplants generally do not need to be fertilized from autumn through winter.)

Miri

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