All of the answers are excellent. Optimum fitness isn’t just muscles (strength), connective tissues (flexibility), or the cardiovascular system. It’s a lifestyle that also includes a sensible nutrition lifestyle….eating sensibly most of the time. And it’s also an attitude that suggests that “I want to live my life this way,” not “I have to live my life this way.” Because the latter won’t persist.
As Kurt suggests it’s a question of balance. Your client is the only one who can ultimately answer the question of how he/she wants to live the rest of his/her life.
Where to start? That’s determined by you expertise as a trainer. What I’ve found is that if you can address one aspect of a healthy lifestyle that your client seems to track into, it opens the door to addressing the other aspects of optimum fitness.