Hiya Louis
Three things tend to have an effect on the performance of electrolytic capacitors, and the chronological age is of little importance generally speaking.
1) The materials used in the capacitors' construction, and more importantly the breakdown and corrosion of the internal structure can have a massive effect.
2) Loss or 'drying up' of the electrolyte on certain types can lead to a measured value far lower than the marked value. I measured a 470uF cap once and found it to be only a few tens of nano-farads!
3) Depolarisation over time. When electrolytic capacitors depolarise they tend to explode or burst as soon as power is applied, very often becoming short circuit and causing large scale damage to the equipment concerned.
So I would say first take a look at the physical state of the capacitor. Any signs of leakage or corrosion? Any dents on the case that could point to internal damage?
If you have access to an LCR bridge, or perhaps one of the modern multimeters with a capacitance measuring range (assuming either of the aforementioned units has a range high enough to measure the capacitor value in question), you could after a physical inspection of the capacitor measure its value and see if it is within the 'ball park' of the value marked upon its casing.
Not to worry if you can't measure the value, you can always rely on observed results when you finally put the capacitor in circuit and test out the supply.
Something you really do need to do though is make absolutely sure that the capacitor hasn't depolarised. On low voltage circuits this isn't so much of a concern, but on a high voltage - high current power supply caution is the keyword.
You can easily re-polarise an electrolytic capacitor by either using a variable voltage supply, or by using a 'Variac' (variable AC transformer) to slowly bring up the supply voltage, while keeping an eye on the potential difference across the capacitor. If you're increasing the AC supply voltage, and the voltage is not increasing across the capacitor, then there is most likely something wrong!
If you don't have a Variac then all is not lost. An old trick used by Radio Amateurs is to add a
low wattage mains voltage filament lamp in series with the capacitors 'hot' end. The idea is that if the capacitor has depolarised then the filament lamp will arrest the current surge and so protect the capacitor and the equipment.
Apparently, if the electrolytic capacitor is found to be depolarised, the lamp not only prevents a minor explosion, but also controls the current flow through the capacitor while it re-polarises. As re-polarisation takes place the lamp will gradually get duller until it finally goes out (indicating no more current flow through the capacitor).
If you don't have a lamp handy you can add a high wattage high value resistor (I've used values from about 2000 Ohms up to maybe 20000 Ohms, with a 5-25 Watt rating) in series with the capacitor, again with the purpose of limiting the inrush of current to the electrolytic capacitor, and again helping to control the current flow through the capacitor while it re-polarises. If you do use a resistor you will need to monitor the voltage across the capacitor, and across the resistor. If the voltage across the capacitor is high and the voltage drop across the resistor is zero (indicating no current flow), then all is well. However, if the voltage across the capacitor is lower than expected, and you can still see a sizeable voltage across the resistor (indicating that there is still DC flowing trough the capacitor), then you may need to leave it running a while longer to see if re-polarisation occurs, or else discard the capacitor altogether and start afresh with a different one.
I have used all of the above techniques over the years (and still do), and have not lost a useful capacitor yet (or had a 'dud' one explode!).
Do be careful though! The higher the voltage the quicker you will die if you mess up! 
Best 73, Mark.
