What is the average life expectancy of Middle-Earth inhabitants?

Using only the movies as source material...

Elves are immortal.

Gandalf says just before he goes to find Eomer: "Three hundred lives of men I have walked this earth and now I have no time." Even if men live only 50 years or so, 300×50 is 15,000.

Bilbo was Eleventy-One at his party, and he was considered "well-aged".

And let's not forget Treebeard, the Ent, referred to "young master Gandalf", so just imagine how old he is.

(This is a partial answer, at least. The movies don't really cover this. There's much more in the books, of course.)

If you're interested in information from the source material:

Dwarves?

Gimli supposedly accompanied Legolas to the West in the year 3141 making him 262 years old. Appendix A in The Return of the King shows the descendents of Durin commonly living past 200 years old. Gimli was about 140 at the time of the War of the Ring.

Men?

Typical of modern humans, but Men of Numenorean blood lived much longer. Aragorn, for instance, lived to be 210, and was 88 at the time of the War of the Ring.

Hobbits?

A little bit more than Men. Bilbo surpassed the Old Took who lived to 130. He was 51 when the events of The Hobbit took place.

Most of the Hobbits I see in Bilbo's family tree (Appendix C) lived between 80 and 100 years old for the most part.

Elves?

Elves are immortal, for they have not received the Gift of Men. (Or the Doom of Men as some Men call it.)

Wizards?

They first appear in the Third Age.

When maybe a thousand years had passed, and the first shadow had fallen on Greenwood the Great, the Istari or Wizards appeared in Middle-earth...

They came therefore in the shape of Men, though they were never young and aged only slowly...

(Source: The Lord of the Rings, Appendix B)

So Gandalf, Saruman and peers show up about year 1000 and Gandalf departs to the West in 3021. It is unknown if Wizards are mortal or how old they can actually be.

No mention is made in the books as to how old orcs can live, assuming they don't die a violent death first, which is really more likely.

All that said, though, the movies deviate quite a bit from the books. In the books, Frodo lives for years in Bag End after Bilbo departs. In the movies, it's scant months.

Bilbo's age of 111 was not all that unusual. It would be like modern humans reaching 100. Somewhat uncommon, but nowhere near unheard of.

What would be unnatural, and part of the rings influence, was how he LOOKED at 111. Note in the movies that when he gave up the ring to Frodo he aged quite suddenly after that.

While I cannot speak to the authenticity according to canon, there is a chart of life expectancy of Middle Earth denizens at this link, which also states Elves and Ainur are immortal.

Tolkiens expanded Books give a more detailed account of life expectancy...

1. Men/Human: 80-100 years (dependent on health, fitness permitting), However Men of Valar loyalty; were blessed with extended lifespans triple that of normal men (300-400 years) Half Elvens who chose mortality, also added to the health of longer lasting bloodlines (Elros lived to be 500)
2. Elves: Immortal: Lifespans are technically "Indefinite" however they are not totally immune to the ravages of time, they may grow weary of life and lose "will to live"
3. Dwarves: 250 approximately, Thorin was 195, Gimli 139 when in the Fellowship
4. Hobbits: 100 years, with 50 being......prime adult age.
5. Wizards: Istari are in actuality "Maiar" thus they are Immortal, however they take on the physical attributes of human form, so it's not seen if they "Age" but apparently do.
6. Ainur (Maiar, Valar) Immortal
7. Ents: Thousands of years (However a majority of their time is spent inert)
8. Eagles: Unknown, but possibly thousands of years old (Possibly immortal as they may be maiar in beast form)