The other side of the coin is that our emphasis on name brand politicians means that they are often too old when they finally get their shot, like 72-year-old Bob Dole in 1996, or even 67 year-old George Bush in 1992, who appeared to have largely retired after the successful conclusion of Desert Storm. Reagan was too old after he got shot, but the country was lucky that his principles were the right ones for the time. Even Clinton at age 54 spent most of his last year in office trying to break Ike's Presidential record for most rounds of golf in a year (103).
Being President is hard, and Obama's youth could come in handy.
There are other reasons to question Obama (e.g., his passionate ethnocentrism, as documented at vast length in his autobiography), but we aren't supposed to mention those, so many are turning to "experience" as a politically correct way to express perfectly appropriate concerns about Obama.