This smells to me like confirmation bias. The actual demographic data shows SF skews older than Manhattan:

    Population by age range, SF vs Manhattan:
    20-24:  3.8% /  7.4%
    25-34: 21.5% / 22.7%
    35-44: 16.3% / 14.6%
    45-54: 14.0% / 12.8%
    55-64: 12.4% / 11.4%
    65+:   15.2% / 14.4%