I've never tried maximizing body potential with an elder.
With a teen, young adult, or adult sim, however, I've managed to max it in about an hour AFTER eating one of those Protein Plates then doing an Epic Workout while Very Energized.
Make the protein plate in advance, then drag it into family inventory where it won't ever spoil.
Good luck! (and let us know if your elderly sim survives and succeeds (^_^) )