"Older" age groups
There are lots of "older" runners throughout the United States.
People who caught the running bug in previous decades are still running. In fact, it's not uncommon in a race for there to be more runners over 70 years old than there are age group awards for them.
So, don't skip "older" age groups because you think there won't be enough runners that age in your event.
After all, many races have no competitors in the 15-19 age group and some don't draw any from the 20-24 age group, either, yet they always have those age groups and they have awards for winners.
Offer age groups as high in age as possible. And don't limit age groups because you want to save money on awards. Runners won't show up unless you offer an age group in which they can compete, so limiting age groups also limits the number of runners in your race.
Five-year age groups are best. Top it off at "80 and over" and always include the same age groups for male and female runners.
Note: Check out "The Golden Years of Running" in RRCA's Summer 2010 issue of Club Running.