Even if SF's population "skews older" than Manhattan's, it doesn't refute the original commenter's anecdote. Older people in SF may dine out less than older people in Manhattan do - perhaps for financial or logistical or cultural reasons.
Or they dine out somewhere else and appear in someone else's anecdotes. Many restaurants attract or repel specific age ranges, often as a deliberate marketing policy: if a restaurant is full of young people it is, usually, a restaurant for young people, for example because it plays trendy dance music fairly loud or because it's close to a university campus.
Manhattan has fewer young people with the means to dine out frequently. It's full of young people who want to be artists, fashion designers, etc... Most people who can afford fancy restaurant meals in Manhattan are older, more established in whatever their chosen career.
SF has a larger percentage of young people who are already wealthy (or middle class) from tech the sector. And thus more young people there dine out, more frequently.
What's the source for your data? While searching, I found this [0] which seems to have some detail by zipcode. (I had thought "Maybe he means in restaurants in the Mission?", suggestion: nope).
