It sounds like your battery, take the cell covers off the battery and look into the cells to see that electrolyte is covering the top of the plates if not, top it up with distilled water.
Geoffers is right, at 5 years old your battery is living on borrowed time, check terminal voltage about an hour after charging and if it is less that 12.5 volts that is a danger sign. While battery is charging look into cell filler holes, any cell which is not bubbling hydrogen gas has probably gone short circuit and is no longer storing power.
Try starting the engine with the headlights on, if the lights go very dim or out altogether your battery is definitely stuffed or terminal connections are dodgy.
Check the terminal connections, especially the positive for corrosion. A quick way to check connections is to put the palm of your hand on each terminal immediately after you have tried to start the engine, if it is hot the connection needs looking at.