Probably your best bet would be to call a couple of roofers for free estimates. They can point out the signs of aging and failing shingles that mean a roof needs replacement.
Because of the age of your roof, it would be wise to get this assessment soon. You don't have to tell the roofer right off the bat how old the roof is - if they know what they are doing, they will be able to tell right away that it is pretty old.
Getting multiple estimates will allow you to get more than one opinion on the state of your roof. From there you have to make the decision. However, just because you aren't seeing water in your living areas doesn't mean there isn't any water getting in. Your best bet is to have it checked.
Lone-Star Roof Systems, Aggie-owned and operated.