Our site has a number of 20+ year old vans, and has just sold a refurbished 1996 Willerby Gainsborough. My parents in law had a 1994/5 Abi Montrose on the site which they sold (through the site) last autumn after they upgraded to a 2004 van on a better pitch
Our neighbour's van has been there about 30 years! As long as they are well maintained and the pitch is kept smart and tidy, I don't think they mind how old the van is...
However, no sub-letting (family and friends only) and all sales over 10 years through the site. Contract is only a year at a time, but I've never heard of anyone being asked to leave except for unpaid fees...