I didn't start organ lessons until I was about Grade 6 on the piano, but there was another reason, I wasn't big enough to reach the pedals as I was only 13. But I didnt do an exam on organ until Grade 5, about a year after I started.:-
1960 Started Organ (age 13) Did Grade 6 piano (LCM)
1961 Did Grade 5 Organ (AB)
1962 Did Grade 6 Organ (AB)
1963 Did ALCM piano
1964 Did Grade 8 Organ (AB)
1965 Rushed into trying to do ATCL organ before going to Uni. Wasnt really ready . Failed it !
Didn't play an organ for several years after that.