The adult stage (産卵期 sanranki) is the longest stage in a Tamagotchi's life span (besides a senior). Adults are the final stage of Tamagotchi before retiring and turning into a senior.
Lots of things become available to a Tamagotchi when he/she becomes an adult. The matchmaker starts visiting them about two or three days after evolution, and they can marry and have babies. In addition, new items may become available for use. In the Entama, Uratama, V4 and V4.5, adults are able to get a job and go to the work section in TamaTown.
On vintage releases, the maximum lifespans of adults may vary depending on their positions on their device's growth chart - for example on the P1, a Mametchi could live to be 21-22, but a Tarakotchi can only live to be 7-8. In most Tamagotchi versions, the sleeping times of an adult will also vary from breed to breed.
Prior to the Tamagotchi iD, the adult stage often lasts until the Tamagotchi turns ten or on some releases up until it turns sixteen. Should it be unmarried by this time, the Tamagotchi will then turn evolve into a senior. An senior will remain on a Tamagotchi toy until it dies or returns home. Seniors can mate with seniors of the opposite gender to produce babies - on all versions except the Music Star where Otokitchi and Ojitchi are capable of breeding, their offspring will warp to Oyajitchi (or on the Tama-Go and TMGC+C, Grippatchi if they are girls, or Tensaitchi Jr. on the Hexagontchi) after the baby hour is up.