Timeless Body does not stop your natural aging
The text on Timeless Body says:
Timeless Body
At 15th level, your ki sustains you so that you suffer none of the frailty of old age, and you can't be aged magically. You can still die of old age, however. In addition, you no longer need food or water.
As a monk, they simply suffer "none of the frailty of old age," which means even as old people, they can still move like they were in their 20's. This makes sense when you consider the amount of old monks who can do flawless kung fu in media. They usually serve as mentors to the younger folk so that their secret techniques don't die with them.
- From "The Invincible Armour" a 1977 John Liu film.
In conclusion: Timeless Body still lets you age, and you may still die of old age naturally. However, due to your timeless body, you don't get the back pain or blurry vision that comes with senescence.
(Heavily adapted from my answer here)
High level spells
The easiest way to achieve effective immortality is to use spells. A wizard has access to several of them once they get higher level spell slots.
Sequester will allow the PC to put themselves into a state of suspended animation during which they will not age. This will allow them to span any amount of time without aging (though it will mean that they are out of action for that time).
Clone will provide the PC with fresh young bodies to use once their older body dies. With enough of these, they can last an infinite amount of time. See this Q&A for more discussion on this.
Wish obviously wish can do anything, including granting immortality. A wish for immortality is unlikely to be granted in a straightforward manner though and would be a good opportunity for the DM to implement some sort of curse or other downside on the PC as a result of their wish.
Imprisonment would work in any of its flavors since the targeted creature does not age while under its effects.
True polymorph can allow you to extend your life by changing into a creature with a longer lifespan than you. Some good potential options: devils are immortal (MM), hags are "virtually immortal", and ancient dragons are listed as living for "over a thousand years".
If your PC isn't a spellcaster you can always try to find them assistance from someone or something capable of casting the spells. For example, Djinni can cast wish as well as some magic items.
Magic items
(Tomb of Annihilation spoilers)
The Ring of Winter listed in Tomb of Annihilation will prevent aging.
Non-spell/item options
At 18th level, druids get the Timeless Body feature:
[T]he primal magic that you wield causes you to age more slowly. For every 10 years that pass, your body ages only 1 year.
Combine this with a long-lived race like the elf or warforged and you easily have thousands of years of life.
An Oath of Ancients paladin at level 15 gets:
Additionally, you suffer none of the drawbacks of old age, and you
can’t be aged magically.
Which seems likely to be able to prevent death from old age as well if your DM interprets death from old age as a drawback.
Warlocks with the Undying Patron also have a similar ability. (thanks @Himitsu_no_Yami)
Epic Boon
A DM can grant the Epic Boon of Immortality to give a PC immortality. This assumes that they have a 20th level PC.
Long-lived races
Races like elves and warforged can live for an incredible amount of time (unknown even in the case of the warforged).
The maximum lifespan of the warforged remains a mystery; so far, warforged have shown no signs of deterioration due to age. (Wayfinder's Guide to Eberron)
Starting as one of these races can get you a good way towards immortality without anything else, and will enhance the effects of many of the options above.
Best Answer
Some oozes are ageless or live a very long time
Notably, Formless spawns:
Shoggoths, for certain:
Colors Out of Space are so alien that we cannot understand if a single creature dies to spawn another, or if they simply duplicate themselves to reproduce, like several other oozes.
Immortal Ichors, which are made of the blood from dead gods. I would say that all oozes with Regeneration, like the Ichor cannot die from old age at all, as the ability simply prevents their death.
Carnivorous blobs could possibly live for centuries, as they are able to hibernate if without food, and ride meteors all the way from Eox to Golarion. There isn't much info about them though.
Oblivions are also said to live for a very ling time, as they usually kill all life in a planet taking centuries to do so.
Radiant essences can live for centuries as well:
Deathtrap Oozes may stay indefinitelly in their trap forms:
Finally, Vorgozen, the kaiju ooze, is still new, though her age is actually uncertain. But she is a kaiju, and like all of them, are ageless.
She also has the ability to hibernate:
Finally, her vague story mentions an "once-flying city", which is a direct reference to the Shory. Their last flying city was Ulduvai, which is the focus of the Crucible of Chaos adventure module, and having ran the adventure, I know the city fell over 5 thousand years ago, making the kaiju at least that old.
But not oozes in general
Other than those with regeneration, there ia nothing in the Ooze creature type that states they are immortal, ageless or imune to dying from old age.