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.
Oxford Living Dictionaries
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.