Original: Both George Martin and Mark Lewisohn speculated that McCartney may have thought of the song when recording began for Sgt. Pepper in December 1966 because his father turned 64 earlier that year.
Simplified: Both George Martin and Mark Lewisohn thought that McCartney may have thought of the song when recording began for Sgt. Pepper in December 1966 because his father turned 64 earlier that year.