I'll just give you my rule of thumb without getting all sciency on you:

1) Cheap cigars, 3 to 6 months seems best and they generally don't get any better after that.
2) Quality cigars tend to improve with aging (multiple years or more) but the nicotine level abates in the process.
3) I just smoke whatever I have when I want it - I rest cigars but don't bother aging them.