Operating Systems' Life cycle

When a distribution is launched, there is a period of time where it is supported, that is, a period of time where the organization in charge of that distribution will fix bugs and update the software. After that time, it is considered obsolete.

Ubuntu: It usually launches a version every 6 months but it has a Long Term Support (LTS) version with 5 years of continuous support.

Red Hat Enterprise Linux: Has support for 10 years.

Debian: Release cycle of 2 years. It always has a Testing version, a Unstable version and the final Stable version.