Probably due to such inconsistency, nowadays this sentence would be rejected as agrammatical by most speakers in both standard and colloquial registers.
This sentence is quite unfortunate; it is too innovative for those who want to keep the standard, and too conservative for those who want it to change. Plus it poses a problem: "Nós" requires the predicative to go into the plural (nós... velho s). But the singular verb, "está", requires the predicative in the singular, so it is hard for speakers to decide whether it should be "nós está ficando velho" or "nós está ficando velhos". Probably due to such inconsistency, nowadays this sentence would be rejected as agrammatical by most speakers in both standard and colloquial registers. It can only be considered acceptable in the popular register - but then it has to get the verb changed, too: "nós tá ficando velho" (or "ficando véio").