If it were a lurcher 9 isn't real old and if one of mine had served me for so long I'd do right by her,arthritis is easily treatable,and won't cost a lot.and may even work with a bit of medication,unless you've already made your mind up.
jmho,some will pts as soon as they can't do a hard day's graft.but if it is in pain it may be the kindest thing to do