I do not know how to differentiate between the "Alter" of some alone and a group of people. What is the plural of "Alter"? Or is using "Alter" as plural customary?
»Alter« as in »what's your age?« is simply never used in plural form in German. [Edit: Okay, looks like some people use the plural form. Still, singular is much, much more common.]
However, the plural form does exist as can be seen by compound forms like »Zeitalter« (historic age) or »Erdzeitalter« (aeons/eons in English/geology):
Der Geologe kennt alle Erdzeitalter auswendig. He knows all the eons (their sequence) by heart.
singular: das Alter, des Alters, dem Alter, das Alter
plural: die Alter, der Alter, den Altern, die Alter
Not much of a difference in form, as you can see.
A common term for age group in German is "Altersklasse", which has the regular plural form "Altersklassen".
If you mean:
das Alter - (i.e. age)
then it really has no plural, though usually composites like "das Zeitalter" or "das Menschenalter" have one.
If you mean the salutation like in:
"Eyy, Alter, hast du mal 'n Euro?"
then "Alter" is not really a substantive, but an ellipsis of "alter Mann".
There also exist
der Alte die Alte
and the plural is
like in "Altenheim" - a home for elder ones.