I came across 'veteran' on another forum.+1
Somewhat in hindsight now, but perhaps that would have been a better option?
In supporting my own suggestion...(I'm not above any human flaw ;-)
To me 'senior' only implies someone being of a certain 'old age'.
'veteran' implies that someone has been active and participating for a longer time and has seen and learned things.
(e.g. has watched C-beams glitter in the dark near the Tannhäuser Gate)
Until somebody thinks of more appropriate qualifications, 'veteran' would have my preference.