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?
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»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):
Not much of a difference in form, as you can see.
According to Wahrig, „Alter“ is uncountable, but Duden gives „die Alter“ as the plural. I agree with Duden, even if the plural is rarely used. The plural certainly is correct for the compound nouns that Ingo mentions.
A common term for age group in German is "Altersklasse", which has the regular plural form "Altersklassen".
If you mean:
then it really has no plural, though usually composites like "das Zeitalter" or "das Menschenalter" have one.
If you mean the salutation like in:
then "Alter" is not really a substantive, but an ellipsis of "alter Mann".
There also exist
and the plural is
like in "Altenheim" - a home for elder ones.