Translation:I do not tell him that I am seventy years old.
yes, It should mean "them" as well.
http://italian.about.com/library/fare/blfare166a.htm (check NOTE)
No, otherwise it should have been "non li dico che ho settant'anni". 'Gli' is just for the third person
So would "non lo dico che ho settant'anni" be a correct translation of "I don't tell him that I am seventy."