The "spawn" period of a demon's life is not long, by the standards of humans.

The first few weeks it spends as a kind of tadpole creature. During this time it is forced to fight for food against other tadpoles.

After a month it is large enough it gains the ability to live outside the pools, and it is transferred to another area that has hazards it must avoid in order to eat, in addition to fighting against its own brothers and sisters.

If it survives this, then Demon Masters send Teachers to collect and begin instruction. Rewards are given to high performers, and punishments to low performers. It is usually around age 6 months at this point.

After 4 years in this "teenage" zone, it is considered an adult and pushed into the main body of Demon's under a Master. It is expected to act and fulfill duties, the same as any older Demon. Many promising demons overstep during this stage and are brought low.

The range of lesser Demons in this capacity can be anywhere from 4 years to hundreds of years old, although older demons that have not obtained rank are often looked down on by their younger counterparts.

Confusingly when a demon changes a human to a "demon spawn" through magic, this is a different condition entirely. These "spawn" are also not true demons. Perhaps it is a term of endearment by the demon creator?

-Notes on Younger Demon progression, by a High Priest