Do you mean oldest or 'most senior', because there's a difference....
The oldest RAF squadron is actually.... three squadrons, because they were formed on the same day, 13 May 1912 - 1(F), II(AC) and 3(F). Source? Various, but go with Jeff Jefford's 'Bible' of RAF Squadrons.
The most senior squadron (source - AHB lists of squadron seniority from 1968 and 1973) is II(AC). Although the sources stated there are 42 and 37 years old respectively, the method of calculating seniority has not changed, and as the top half dozen squadrons at the top of the list (in order, II, 1, 6, 4, 14, 8) have remained extant since those lists were drawn up, the pecking order has not changed.
Edit - Wholi - 2 and 3 were created by renumbering No 2 (Aeroplane) Company of the RE and splitting the assets between the two new formations at the same time as 1 was formed from 1 (Airship) Company, RE; this, of course, was the result of the RFC coming into existence and taking over the RE's assets.