First signed it age 15 or 16 when I joined the Royal Observer Corps (well someone had to) and we were asked to re-sign every 2 or 3 years. Subsequently re - signed on being appointed a CI in the ATC and when being commisioned in the RAFVR(T); I think I had to when I relinquished my commision too.
I certainly had to sign it on joining NATCS (now NATS) as it was part of the Board of Trade and we were civil servants.