I have no personal opinion on the maturity of Ruby but maturity is not only relative to age. There are many potential indicators of maturity; some may matter to you and some may not.
- Stability
- Number of bugs in the stable release
- Number of installations
- Size of its community
- Number of releases
- Does it follow a formalized release process?
- Does it follow a formalized change control process?
- Does it make backwards compatability a priority?
- Age
- Insert whatever matters to you here...
- etc...