Some adults are said to have a mind of a child, but what about the rest of the population? What milestones should the rest of the age groups have to belong to a certain mental age?

Good question

Well it really is simplified into personality trait, one with the mind of a child is mentally immature whereas a mature individual or “normal” person is more settled in adulthood.
In psychological terms intellect and knowledge are not age dependent but rather the effects of life experience are. The mind of an adult is more conditioned to respond to the responsibilities and events involved at that certain stage in life

@HolographicUniverse In other words, I was an adult around age 11?

Personally, I use roughly the same guidelines as I would to differentiate sentient and non-sentient life. Basically, an adult human can extrapolate the future, something children and animals cannot. They can also make informed decisions and handle the consequences of their actions. That second one means that many 21st-century Americans do not fit my definition of “adult”.

They say men are happiest at the (approx.) age of 37. I just read this somewhere.

We are always going through developmental stages. As we change and age, our needs change. The needs I have now for e.g are different to those at 25, 35 and even 45.The elderly or geriatric are also continuing on their own developmental stages. Emotional maturity is not always acquired by a particular age. There are various factors for this. Or it can be transient. Meaning at times we are at times we are not.

The highest mental levels are subjective. For example one mind seek absence of ego.

One of the key criterion is the ability of pausing before acting when emotions are intense.

The length of the pause correlates with the mental age. For a typical average 35-year-old adult the pause should be considerably longer than for 20-year-old adult, say 1.2 seconds versus 0.8 seconds. The highest mental levels might involve pauses of 2–3 seconds or longer.

Tell me why we should care about milestones, and I’ll give you all the milestones you want. Unless you have a question driving this and a purpose for it, all answers are arbitrary. Is there some way of knowing objectively where people should be at a particular stage of life? If so, that will give you the answer to your question.

Personally, I do not think there is any objective way to determine where people should be. You might be able to do some tests and describe the average development on certain criteria at certain ages, but to move from that to normative statements would be a mistake, I think.

We need flexibility, not milestones. We need roads that work like those in “The Phantom Tollbooth,” not like the Autobahn. We’re humans, not cars. Developmental milestones do more harm than good. They keep people from relating to each other as people.

I assume it all depends on the group that you surround yourself with?
If a child is surrounded , with only mature adults , then that child would have an adult maturity about him, right?
Same as a child growing up with childish people , then this child would have “arrested development issues”?( unless of course..this child has other personalities in his environment , that he could emulate and learn from)?

I have seen single child families where the child mirrors what he has learned in his behaviour expectations etc.