Just came back from the Rally (in Syracuse) and it seems to be the consensus that you replace ties at 5-6 years. My tires were made on 9/07 so I am ready to bite the bullet. Got a quote from a local, ...
5-6 years is a bit extreme, but many will post that they start losing sleep at 5 years old. I'd rather spend my money elsewhere. If my tires pass inspection at 7yrs, I'll have them checked annually until 10yrs, then replace them.
Of course, visually inspect them regularly, regardless of age and keep the correct pressure all the time. If a visual problem is discovered, it's time for a change.