The edges of the leaves turn brown/ black then rest of leaf turns black and falls off ??
Why is this happening?
Indoor plant 20 years old

Something about its growing conditions are no longer working well for the plant. This weakens it, and makes it susceptible to various infections.
For starters, I can see that it no longer has overhead light, only a little side light from the windows. The leaves are all at the top, and they need light to do well.
This is a problem with keeping this type of plant indoors, they out-grow the space available. I don't know if it will come back if you prune it, maybe we'll hear from someone with more experience.
Can you add a picture of the pot? Have you ever re-potted? How do you water?
I'd say you've done well to keep it going this long, but it doesn't look well now.

As roots fill the pot and soil materials break down into increasingly finer particles, essential resources like oxygen, water, and nutrients, become increasingly difficult for the plant to assimilate. Difficulty obtaining essential resources results in conspicuously slowed primary growth (extension of stems/branches and foliage) with foliage concentrated (again, conspicuously) at the apex (growing tips) of stems and branches where elongation occurs.

The Madagascar palm offers an exceptional opportunity to observe the effects of root congestion and inadequate resources.

The stem of this plant ^^^ reveals it has been up-potted twice. Notice how the thorns become increasingly closer together from the soil line up to the midpoint of the lower stem. The radical change/increase in the distance between the thorns at the middle of the main stem is the result of potting up, which partly eased the stress of root congestion, increasing the rate of elongation. Again, viewing from the midpoint up, you'll notice the vertical space between thorns again begins to decrease until the second repot, after which the plant branched. The branching might have occurred as the result of the reduced stress due to the up-potting, or, it might have been the result of the grower pinching the plant (removing the stem's apex) to force branching.

Additionally, if you haven't been fertilizing regularly, deficiencies of mobile nutrients (nitrogen, phosphorous, potassium, magnesium) force the plant to rob older leaves of nutrients which serve as building blocks the plant needs to produce new foliage. Once the older leaves have been robbed of nutrients, the plant sheds them.

Finally, making sure the intervals between waterings are appropriate are also essential to your plant's well being. Watering too often can limit root function or wreck root health, either/both of which limit the volume of foliage the plant is capable of supporting.

With additional dialog, it's a very good bet the actual underlying cause can be determined more precisely. There just isn't enough information at this point to determine a singular impetus behind the unwanted symptoms. We'll look forward to additional input and any questions you might have.

Al

Thank you for your thoughts on my Palm tree.
I will try to repot it to ease stress on the roots ( although that will be quite hard to do with all the thorns) . I have never given it fertilizer and it's always done well. This summer we put it out on our deck where it was in a very hot spot for much of the afternoon and that seems to be when the browning of the leaves occurred. I tried to move it to a shadier spot to see if that helped but the browning continued.
On another note:
Is it possible to chop off a piece of the palm at the top ? Will it regrow the leaves ?It is getting too tall for our sunroom.
There is a local conservatory nearby that said it would take it but I would have to bring it there and I can't fit it into the car!!

This isn't a palm tree it's a succulent. It's normal growth habit is to shed all the leaves in winter then regrow them

Or, can theg7 top be chopped off and rooted?

@Gina1960 thank you for your response but uncertain about your comment ". This is a succulent "
?
Does this mean it isn't a Madagascar Palm or that that type of plant is a succulent?

It does shed its leaves every winter and they do grow back in the spring but this year the leaves have this brown edge before they drop off and they turn dark brown which isn't normal.
I googled it and it said it is a succulent and a cactus.

Madagascar Palm is just the common name, most likely because it resembles a pam tree but it's not related to actual palm trees. It's a tropical succulent plant.

Related to Plumeria, Adenium, Oleander, Vinca and Catharanthus.

To add to what others have said, there's nothing abnormal about the leaves turning dark colors before they drop, and it is entirely normal for them to fall at this time of year.

I would be concerned about the plant getting so tall that the leaves no longer receive sunlight. This is more or less an inevitable outcome for this plant indoors if it manages to grow as tall as yours. Keep up the care that you've been giving for the last 20 years, because it seems to have worked out very well, but bear in mind that this growth cannot go on indefinitely indoors. Your plant is very close to the limit in terms of how tall it can get before it begins to fail in that situation. It is a tree, after all.

You can restart it from a short-stemmed cutting (best done in spring at the start of the growth season) if you really are intent on keeping it alive (instructions upon request). But I would just celebrate its majesty in the moment for as long as it lasts.

@Baja_Costero
What happens if the trunk is pruned down much shorter? Will it branch/regrow from the stump? Or will this kill it off?
I'd like the details--what's the best way to re-start the top cutting?
Will the in-between bare trunk segments sprout, as some (smaller) succulents do?
I'm also dealing with one about to hit the greenhouse ceiling.
Thanks!

They will branch from the top of the stump after they are beheaded. This behavior is presumably faster and more vigorous the closer the cut is to the tip. So cutting near the base of an old plant may be dicey.

I have beheaded plants on purpose in order to force branching (removing the top inch or so) and I was pleased with the outcome. The response is very similar to what happens when they flower, when the growth point has turned into an inflorescence. A headless plant will likely not be headless for long, given permissive conditions and reasonable vigor beforehand.

Starting Pachypodium cuttings is not a whole lot different from starting cactus or other succulent cuttings. You have to give them time to heal before potting them up. Some cuttings will leak sap for a while and this may create a big black stain, so put something underneath while they heal.

I like to go in with nail scissors and remove any spines around the bottom of the cutting at the base. Just the spine. This is because as the base of the cutting heals, it sort of shrinks inward, and as a result a spine at the bottom of the stem would start to point downward and get in the way down the road.

Pot the cutting up in a relatively small pot (not a deep one) in fast draining mix (like 50% pumice or gritty equivalent). The base of the stem should be in contact with the soil but very little stem should be below soil level, the bare minimum needed to keep the cutting upright. Maybe put a rock on either side or stake the cutting to help with stability while it is rootless. But don't bury the stem too far.

Water with restraint at the beginning, more or less when the soil has gone dry or almost dry. At some point you will begin to note activity on top, and that is a sign that you can increase watering. These plants are thirsty during the warmer months when they have roots... I water rooted, established Pachypodiums twice as often as the average succulent while they are in leaf.

Indoor plants are going to require and consume much less water than a plant out in the sun and the wind, so don't go crazy with the water in the months after you pot up a cutting, if it will be indoors.

Segments may root and sprout new branches. I've never tried but I would imagine it's quite possible, if conditions are entirely favorable. Make sure you keep a upright orientation (not upside down).

Above all you will get the best results (forcing branching and rooting cuttings) if you take action at the start of the growth season, or close to it. This time of year would be the absolute worst possible, so think and plan and prepare, then execute in six months or whatever.

Thank you Baja, this is very helpful.