As one of the founding squadrons of the RAF from 1918 I am not sure how you can get anymore senior. Formed 31/12/1916 as number 7 (Naval) squadron. You could go for a lower number but of course all the really low numbers up to about 144 are ex-army.
Still if you chuck some water about the landscape and have a really high number you are good to go forever. Well almost, only 5 years disbanded since 1943!!!!