Precisely. The potential advantage older people have is that their energy, if they apply it, tends to be far more focused and therefore effective -- working smart instead of hard. As I said, we've hired grandmothers already at their retirement age at high-tech startups, and I know of several others who are still in the game too. They aren't bouncing off the walls like a 20-something, but they are still very much in the game and far more effective than some of the young people that get brought in. Most of "acting old" is about coasting and doing the minimum to get through the day.
It's more than that. It's the complaing all the time. It's the looking at you like that's a stupid idea or pursing your lips or making some kind of remark like okkkkkkkkkkk or the sighs and getting up slowly. It's frowning and not liking change. When managers dread approaching you, your work days are usually limited IMO.