This phrase is an appeal to widespread knowledge and consensual values about how a person of a certain age should act. In a time where both widespread knowledge and consensual values are in decline, you would think that sentiments like this would seem oddly dated. Do parents still say things like this?
There are reasons to undermine the whole structure of the question. The emphasis we have now placed on the unique individual clearly have no use for “age based norms of behavior.” There has to be a way “one should act” at a certain age for the whole “act your age” thing to make any sense. That way is under serious threat.
Another reason is the rise of therapeutic language and the perspectives on human behavior that surround it. In an episode of Doc Martin that Bette and I watched recently, a dreadfully willful and undisciplined kid goes around the village scratching the paint of cars parked on the street. The father explains that the boy is just exploring his ambivalent feelings about authority. 
On the other hand, excuses like “boys will be boys” apply the “act your age” dictum in the other direction. The shield here is, “They ARE acting their age.” So…this kind of behavior is so common among boys that age that it should be expected and therefore not “abnormal.” If it is common, it is normal. If it is normal, it is acceptable. That goes downhill fast.
So we can, as these paragraphs indicate, undercut the whole rationale of “act your age,” but that’s not why I introduced the topic. I introduced the topic because I am an old man and I live in a senior center and the implications that flow from “act your age” are a different kind of thing entirely here. How does an old man or an old woman go about acting their age?
I’d venture to say that most of the men who live at HPP  have had sports team experience. There is a kind of banter that is normal in locker rooms. People who use that language are accepted as part of the team and those who don’t have an extra hurdle to get over. Trust me on that. If a group of men at HPP found themselves using that old sports-based language, it would be disapproved of—and not just by the women.
But we are looking here at the basis for that disapproval. Would these men be admonished to “act their age?” I think so. They could be reproved for being racist or sexist or ageist or whatever young men delight in, but that would be a morally heavy charge and we do all have to find a way to live together here. That is why I an age-based criticism might be chosen. “Sure high school athletes talk that way among themselves, but you aren’t high school athletes anymore.”
I think gendered patterns of interaction might meet the same fate. There are, roughly speaking, three groups at HPP as they bear on this question. There are men and women who still notice and appreciate gendeer differences. There are men who prefer the company and conversational style of men and women who, similarly, prefer to be with women. And there are those for whom noticing the difference at all has become burdensome.
If the norm is that old people just don’t notice (much less appreciate) gender differences, then an old man or an old woman who does notice and who does appreciate them, could be said not to be acting their age. “Age appropriate behavior” would then be defined as not noticing or not caring. It is “what old women and men are like.” Furthermore—once it has been formulated as a norm—it is what old women and men should be like.
That would mean that behavior that would have been unlikely even to be noticed at an earlier age—say the last third of a career—will be discrepant and worthy of comment. This could be taken as an affront by any man or woman who deny that a mutually enjoyable recognition of gender differences that have been treasured by both parties over a long life should be discarded on the grounds of age alone.
Once you move out of the developmental context—the “you shouldn’t be sucking your thumb any more now that you are X years old”—context. The whole standard gets a little fuzzier. Age-related norms are not as clear and the agreement about them begins to fray at the margins. There are so many other ways of criticizing behavior, that it seems a shame to use one so vulnerable to abuse.
Maybe “act your age” is not the kind of thing that should be said to old men and women.
 And, to raise another serious but unrelated problem, the father tells Doc Martin that Martin’s car insurance will pay for the damage so he has no reason to be offended by the child’s behavior.
 Just a convenience. Holladay Park Plaza in a senior center in Northeast Portland. There are roughly twice as many women as men and roughly ten times more Democrats than Republicans, especially recently.