I like *mature* or *proven*, but consider ***refined***:

> It is rather old, but ***refined*** technology.

I believe it gives a stronger connotation of something that was actively improved over time.

> **1.2** Developed or improved so as to be precise or subtle.<br>
<sub>[Oxford *Living* Dictionaries][1]</sub>

The strength of a *mature* or *proven* technology is the implication that it has been in use for a long time with few faults. It doesn't necessarily mean that it has been actively improved over time.


  [1]: https://en.oxforddictionaries.com/definition/refined