In Star Wars, does anyone know how old Yoda is? Is it mentioned in any Star Wars movies?
Straight from The Return of the Jedi
YODA (tickled, chuckles) I do, yes, I do! Sick have I become. Old and weak. (Points a crooked finger) When nine hundred years old you reach, look as good you will not.
Additionally, in case you think it's a case of hyperbole, the Star Wars Databank mentions that he is at least 800 years old.
Yoda was a legendary Jedi Master and stronger than most in his connection with the Force. Small in size but wise and powerful, he trained Jedi for over 800 years, playing integral roles in the Clone Wars, the instruction of Luke Skywalker, and unlocking the path to immortality.
An archived blog entry on starwars.com written by Leland Chee lists the major characters' birth years. It states that Yoda was born in 896 BBY, and that Return of the Jedi took place in 4 ABY, so Yoda was 900 years old when he died:
~ -600 Jabba the Hutt
~ -200 Chewbacca
-102 Count Dooku
-92 Qui-Gon Jinn
-82 Palpatine [47BrS:8:11]
-72 Mace Windu
-66 Jango Fett
-57 Obi-Wan Kenobi
-46 Padmé Amidala
-41.9 Anakin Skywalker
-32 Star Wars: Episode I The Phantom Menace [3:4:14]
-31.5 Boba Fett
-31 Lando Calrissian
-29 Han Solo
-22 Star Wars: Episode II Attack of the Clones [13:5:21]
-21 Wedge Antilles
-19 Star Wars: Episode III Revenge of the Sith [16:5:23]
-19 Luke Skywalker & Princess Leia (Luke born first)
-18 Corran Horn
-18 Mara Jade Skywalker
0 Star Wars: Episode IV A New Hope
3 Star Wars: Episode V The Empire Strikes Back
4 Star Wars: Episode VI Return of the Jedi
9 Jaina & Jacen Solo (Jaina born first)
10 Anakin Solo
26.5 Ben Skywalker
This out-of-universe statement indicates that Yoda had reached literally 900 years old when he died (he didn't exaggerate when he told Luke that "when 900 years old you reach, look as good you will not"). Since Leland Chee is responsible for Star Wars canon (even post-Disney), this is canon information.
Just to a give another answer from a certain point of view... Legends tells us that Yoda is indeed exactly 900 years old.
- Born in 896 BBY
- Died in 4 ABY
However, my hunch is that this was done to match the line from Return of the Jedi...