I don't know, I think they mean the same thing, but "getting old" has the connotation that it's towards the end of your life. That is, a baby is constantly aging into a slightly older baby, but it's not getting old yet. A thirty year old ages into a thirty one year old, but unless you're a teenager you wouldn't generally say they're getting old yet. But someone who's retiring and moving to florida? They're getting old.

Of course you might also say "you're getting old!" to say a fourteen year old cousin that you haven't seen in a few years. It depends on context. But I don't think they literally contain the same information except for syllables.

So I think jlduggers pedantry is misguided in this case.