I don't know about the oldest army but I know the oldest unit is the Pontifical Swiss Guard. The unit was founded on 22 January 1506 as the bodyguard of the Pope and to protect Vatican City. It is now 508 years old.

Could someone explain to me what is meant by a 'standing army'? I understand that it means they are always in the army, even in times of peace, but I've heard that these 'professional armies' didn't appear in Europe until relatively recently. Why wouldn't Roman legionnaires or the like be considered part of a 'standing army' if they signed long (I think 8 year) contracts?

I think we need to concretely define "standing army" for this.

For example, I think we can all agree that the current US army constitutes a standing army, at least in part. Troops live on bases with the rest of their unit, their weapons are housed within the base complex, etc

It becomes a little more nebulous when we bring up off base housing. Now the troops are living away from their weapons and units. Their full-time job is "soldier" though, so they are available for military duties at all times.

The national guard might also fit the bill. Like off base housed troops they live apart from their unit and weapons but they have a rank in the organization and an established route to activation with gear awaiting them on a moments notice. They have other work, however, and while they can be pulled away from that at any time, economic considerations temper the state's ability to respond with them instantly.

Now we get into militia groups which are rather like the guard. Again, rank structures persist even in periods of inactivity and equipment is maintained. Personal ownership of weapons is more common. Heavy weapons are less common.

Now, finally, we are to the kinds of units we can all agree are not standing armies. These are the ones that exist only on paper, perhaps with a professional officer corps, which are created out of whole cloth during a mobilization. Perhaps not institutionally, but at least personally, the vast majority of the combatants in most pre-modern wars fall into this category.

Where on the continuum are we talking?

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