A Chao’s life lasts for around 5 years and then it reaches the end of its life. The age of your Chao is shown in the Medical Chart with the Chao Doctor (in the Kindergarten).
A Chao Year lasts about one real life hour on the Dreamcast games, and around three hours on the modern games. If you care for your Chao extra well, it’s years will take longer. Likewise if your Chao is unhappy and get hit alot, it will take shorter to age and in turn, die quicker. Also note, on the Dreamcast Sonic Adventure 1, when a Chao successfully breeds with another Chao via eating a Lifenut, its lifespan also decreases.
If your Chao had a high enough happiness stat (this is a hidden stat– greater than 30 for SADX and greater than 50 for SA2), it will reincarnate and leave an egg. If it didn’t, then sadly your Chao will not reincarnate and will be gone forever. When your Chao starts going into a cocoon about to die, in the modern version of the games the cocoon will be pink coloured if it is going to reincarnate and grey coloured if it is about to disappear forever. On the Dreamcast and SADX Steam version, the cocoons are always white regardless of the outcome.
If your Chao reincarnates, it will keep its name, race badges, but will lose all but 10% of its stats (except SA2DC, where it resets to zero!). Aditionally, in the modern games which have stat levels and grades- the Chao’s levels reset to level one, and its grades remain the same.
Dreamcast Chao Doctor
If your Chao is 5 hours old or less, The Chao Doctor will say “this Chao is growing”. More than 6 hours will result in him saying “this Chao has had a long life”.
There is a glitch on the Dreamcast version of Sonic Adventure 2 where it was said that if you take your reincarnated Chao to the Chao Doctor - it could crash your game and corrupt your save file!
This is explained in full on the glitches page. If you play Sonic Adventure 2 on the Dreamcast be sure to read this!