I think that the general consensus here (and in science, which is where this comes from in the first place) is that children's developmental steps are not tied hard and fast to their age. Just as importantly, certain social expectations depend entirely on your parenting - eg: at what age should opposite-sex siblings stop bathing together?
I can't even imagine a parent putting up with a kid's insistence that they are old enough for an activity for long enough for age (in years) to be a real deciding factor. Can we discourage or at least warn against asking questions that start with "at what age should..."? If the question is about possible developmental delay, they should ask about a possible developmental delay, rather than asking "at what age should a child start potty training" when the real problem is that their 4 year old refuses to start.
I understand in crystal clear detail the motivation for asking "when the hell is developmental step X going to happen already, dammit?!" but the fact of the matter is, that noone can put a number to it, and in the end, it's all about the child's individual development. These sorts of questions usually have answers that begin like "well, there is no age, really...".