Contiki FAQ(contiki.com)
Can you go on a Contiki trip if you’re over 35?
Contiki trips are designed exclusively for 18 to 35-year-olds. This is because it gives young people the chance to travel (and make unforgettable memories) with a group of people who are all the same/similar age.
HOWEVER, if you’ve travelled with us before, and are aged between 35 and 45, you can recapture the magic of Contiki travel on one of our limited edition Reunion trips.
How does the under 35yo come into this? Quick look and couldn't see anything in the T&Cs.