When parents are new to a way of parenting, whether it's parenting at all, or a change in parenting choices, many times, there is stress involved in "figuring out" whether things are "working." Whether their child is "on track," or either "ahead" or "behind."
The dominant culture would have us believe that there are milestones in knowledge and behavior associated with every age, and, specifically, with every grade in school. People with strong attachments to schooled ways of thinking, or who are being criticized by friends or family, often worry about whether their child knows what they should know, or is doing what they should be doing.
While there is a very general "guideline" for child development, the truth is that every person is an individual, with their own timeline, which may or may not fit the ones in the textbooks. Not only do they have their own long term timeline, but what a person is able to do or recall with comfort and ease changes all the time, from day to day, or even hour to hour.
The question becomes not "What should my child be able to do at this age?" but "What is my child able to do right now?"
If a person is usually able to handle certain situations with ease, but they are over tired, or sick, or frustrated, or distracted, or pretty much any potentially overwhelming emotional state, even more "positive" ones like excitement, they may well NOT be able to handle that very same thing at any given moment. It doesn't matter what they were able to do before, what matters is what they can do NOW. Being angry or frustrated that they can't do something doesn't help the situation at all, and, in fact, may easily make things worse.
This is as true of adults as it is of children. There is a constant ebb and flow.
The key is to pay attention, and notice the emotional environment as well as whatever else is going on.
Observe what a person IS able to do, rather than what you expect them to be able to do, and go from there.
Whether it's being able to read, or being able to sit still, people are able to do things at all when they are ready, and will learn to do them at their own pace, and retain that ability or not, according to their own nature and the environment in which they live.
Notice what helps, what leads to more happiness, more love, more self confidence, more fun, and what doesn't.
Work with each other, not against each other or preconceived ideas of what a person "should" be able to do.
Even if there is a real "developmental delay," a person still can only grow from where they ARE, not from where they "should be," anyway. Respect who they are. Love who they are. Move forward. That's really how life works, for all of us.