The traditional view, according to the explanation that I've seen, was that while few individuals reach a lifespan of 99 years, even fewer make it to 100 years and beyond. So, regardless of how the agreement might have been labeled, a "lease" of 100 years or longer was effectively viewed as an outright sale of the property. The 99-year term was used to convey the message to all concerned that the contract was indeed a temporary arrangement, even if the original parties to the agreement might not be around to see its conclusion.

This may have to do with common law concepts the "Rule Against Perpetuities" that is too complicated to explain.

"Rule Against Perpetuities" thanks a bunch! that really helps.

Does anyone know of a legal source that answers this question?

Some state law allows lease terms up to 99 years. So having a lease greater than 99 years in those states would make that lease null. Most states do not have this law so a lease could can be several hundred years but then again, this could create conflicts having perpetual lease agreements. It's not based on how long a human can live since a lease holder can sell their proeprty with a leased land but simply a State Law that limits the max lenght. Agricultural leases are less in most states.

Here is the real reason....from www.straightdope.com/classics/a1_138.html The 99-year lease is apparently a by-product of an old English custom, dating back at least to feudal times. In the Middle Ages, extended leases were made for a period of 1,000 years, but as the Renaissance approached, the figure was reduced to 999 for reasons that today aren't entirely clear. In Henry IV, Part II, Shakespeare bends a metaphor around the thousand-year lease--"Now I am so hungry, that if I might have a lease of my life for a thousand years, I could stay no longer"--so the practice must have survived into the 17th century. But a contemporary of Shakespeare's, Sir Edward Coke, is already speculating on the origin of the 999-year lease. The thousand-year lease, he thinks, might have at one time been ruled fraudulent by the English courts--a lease that long was really a sale. If such a law existed, the landowners would avoid it by setting a term of 999 years, the loophole hardly being a modern invention. But no record of any such ruling exists. The 99-year lease was largely an American invention, the hubba-hubba colonists apparently having no patience for the long-term view, and was probably worked out by analogy to the traditional 999 year figure. But not all of the experts agree. John Bouvier, writing in his 1839 Law Dictionary, the first legal reference book published in the United States, offers a different explanation: "The limit of 99 years would seem to be connected with a somewhat arbitrary estimate of 100 years as the probable estimate of a man's life. Leases for years are in their attributes, evolution, and history, a sort of middle term between estates-for-life and a tenant-at-will. For this reason a period little short of the duration of the life of man was devised so that the lessee might reasonably build or lay out money for the property." Other speculation ranges from the cabalistic, focusing on 9 as a mystic number, to the hierarchical, 99 years being the approximate period covered by three generations. The hard-nosed approach suggests that the government once levied a higher tax on leases of 100 years or more, but there is no concrete evidence of this. You are perfectly free--this being the land of the same--to draw up a lease for whatever term you wish, a millenium or twenty minutes. The 99 year figure has certain obvious advantages, being a nice, solid number to satisfy the bankers, but still being wide open for all practical purposes.

I'm not a lawyer or student of the law, so the following is dredged from my own deep fund of ignorance. Some 40 years ago I lived in Honolulu for a time, working for a firm of land surveyors. The boss told me that hardly any home "owner" in Hawaii had fee simple title --- they were almost entirely on 99-year leases. The reason, he said, was that from the earliest days of the American occupation of the Kingdom of Hawaii, the big American corporations, variously known as the Big Five, sometimes the Big Six, etc., began buying up all the Hawaiian land they could get their hands on. And they still own it, and are reluctant to let even a square yard get out of their possession. You've heard of the Golden Rule? "He with the gold, rules." Perhaps the same thing applies here, with what might be the Land Rule: They with the land, rule. Hence the 99-year leases, in this particular set of islands. Any present or former Hawaii residents who can expound, clarify or correct?

Actually, the earliest instances of leases for 99 year are much older: There is
an Egyptian papyrus of the year 288/287 BC in which a priest leases out his
office for 99 years (Papyrus British Museum 10240), and other examples of this
period (3rd-2nd century BC) show that also fields were leased in that way. I
had hoped that someone could tell me how the Egyptians (or probably the Greek,
since at that time they occupied Egypt) had come up with this number, but it
seems that no one really knows ... As to why, this was a neat method to avoid
sales taxes while having approximately the same effect - but that doesn't
explain the number 99.

ask yourself why do product at the store cost x dollars and 99 cents. 99 makes
it just low enough to accept but not to high to refuse.

This is annoying though. Shops seem to forget that customers can usually do
rounding for themselves! Then again, there are probably some statistics that
back up the "phenomenon" -- maybe it appeals to the subconscious mind of an
impulse buyer...

The answer is that it was a tax evasion scheme to avoid paying feudal dues
incidental to selling land. The Statute of Uses of 1536 was a response to
crafty common lawyers, who set up a system whereby the owner (A) would bargain
and sale his land to another person (B, usually his lawyer), who would hold
legal title of the land "for the use of" C. This could be done secretly and the
goal was to avoid the public ceremonies of homage and fealty and livery of
seisin, which triggered feudal dues to the King. The Statute of Uses executed
all of the uses and fee simple vested with the beneficiary. At the same time,
Henry VIII also passed the Statute of Enrolment (1536) and established the
Court of Wards (1540) in order to register and record the titles to these fees
and collect his taxes/feudal dues.
About 1621, Sir Francis Moore invented the bargain and sale for long leasehold
terms, which is the answer to your question. The Statute of Enrolments did not
affect bargains for a term. By its operation, the bargainor is seised to the
use of the termor, and the termor qcquires the legal term under the Statute of
Uses (because a bargain and sale involves a 'use') without having to enter.
Thus it was possible to convey secretly (and avoid feudal dues such as fines on
alienation) by using two deeds: (1) a bargain and sale for a term, followed by
(2) a release of the fee to the termor. Neither had to be enrolled under the
Statute of Enrolment and neither need actual entry on the land for its
completion. Hence the 99 year lease.
The property on which Yale is situated is an example of the 999 year lease.