A monster's age is a measure of its lifespan and determines how long it can continue to be trained.
There are multiple stages of a monster's life:
Infancy: The first stage of a monster's growth from the time it is generated or combined. During this period, stat growth will be at its slowest (2-4 per training, comparable to Senior growth) and the chance of failure at training is at its highest.
Adolescence: Once a monster enters adolescence, the stat growth will rise (5-7 per training), getting closer to its Adult levels. Failures will drop off around this point.
Adult: One of the two visible stages of growth, Adult is the point at which the stat growth will be the highest (6-9 per training). The game recommends focusing on training during this stage.
Middle age: Once the adult stage indicator vanishes, stat growth will drop back to mirror the adolescence range.
Senior: The other visible stage of growth, Senior is the last stage in which a monster can be trained, battled, or sent on adventures. Stat growth drops to minimal so it is best to focus on battles and adventures at this stage. The game cautions against training a Senior monster unless it has "lots of energy" (how much energy this is isn't specified).
Retired: Once a monster reaches its maximum age, it cannot be trained, battled, or sent on adventures. At this point, the only option is to freeze and combine it with another frozen monster. Monsters must reach this stage in order to get a black star on its Wiki entry and to be counted toward the number matured.
How long each stage lasts, as well as how long its total lifespan is, depends largely on the type of monster, although it can also vary by training methods, a small random variation, and hidden factors (for example, training a monster until it runs out of energy during its Senior stage can shorten its lifespan.)
Where confidence (C) is a number between 1 and 10 (represented by 1 bar for ever 2 numbers and rounded down; for example, a confidence of 5 displays 2 bars on the info screen), stat gain per training is as follows:
- Baby/Senior: 0.5C
- Adolescence/Middle Age: 0.75C
- Adult: 1C
- Amazing! stat gain: 1.5C
- Failed stat gain: Always 1