Battery age isn't always a good way to check a battery. You need to check the voltage after manually charging it, and then let it sit for a day, and check it again. If it doesn't loose voltage, it is safe to say that all the cells are working as they should. The second thing that needs to be checked is the alternator output voltage. This is a little more difficult, unless you have the proper equipment. There is a slight possibility that there is a damaged diode in the alternator, or it could be something simple as a loose belt, causing the belt to slip when the vehicle calls for full output of the alternator. Depending on the alternator that is installed on your car, the fix can be inexpensive, or very expensive. If you have the 130 amp alternator, it will be expensive, and parts are difficult to find. What sometimes is even more difficult, is finding someone that knows how to work on that alternator. A good alternator repairman will have no trouble fixing it, if you can find the parts. Problem is that it has to be disassembled and each diode needs to be checked to see if it is good or bad. When mine went out on my 1969, it took me almost 3 weeks to get the parts needed from a few different suppliers. No one supplier had all the diodes that it required. I did luck out with a shop in California that Kevin O'Connell recommended, however, I recently learned that the shop had closed. Kevin has no idea what became of the inventory.