Granted, it's possible to be 60 years old, foolhardy, and out-of-touch -- just as it's possible to be 30 years old, whip-smart, and tuned in.
But in general, voters value experience, and you don't amass experience without aging.
I agree with you, though, about term limits. They're a good way to balance voters' desire for experienced candidates with the need to inject new blood from both sides of the aisle.
Unfortunately, if there's one thing American voters stupidly fall for beyond incumbents, it's nepotism, so if term limits were in place, I predict we'd see more family dynasties in Congress.