Central Air Conditioner Unit Help

by John
(fort smith,ar)

Our central air seem to work fine until the temp goes over about 92 and the humidity goes up, we live in Arkansas.

Some rooms are at 80 degrees and some at 70, one sits around 74.

We have been told the air coming out of the vents is at 58. We have had the Freon checked too.
The unit is at least 15 years old and last summer we had a new fan and motor installed.

Is the unit not blowing hard enough to keep the rooms cool, is it just too old. How do I know, what should we do next