@Pascal's answer covers it very well. This is just background information.
There's no one right answer; it depends on the child as much as anything else.
There are many thousands upon thousands of studies on adolescents and independence, some dating back many decades, and there is so much contradiction in the results that it's difficult to come to a firm conclusion about what's normal and helpful to future relationships in the acquisition of "independence". A lot of the confusion stems from how one defines independence, which is difficult to define. So I'll try to answer, but know I'll be leaving out almost everything.
Is it appropriate for children that age to be that independent?
Reinterpreted, it is perfectly normal for young adolescents to start keeping secrets from their parents and doing things without telling them at this age.
On the one hand I feel that the anger of the one mother would certainly instill a feeling of not doing something like that again in her son...
Not an either/or situation. Yes, it might make the child think twice. Or, if this is the usual response to autonomy, it might demonstrate a need for increased stealth and might hurt future communications, i.e. might damage the parent-child relationship, and future interpersonal relationships. But relationships and behavior are not based on isolated incidents.
Or should the understand that they at least have to tell their parents first?
They did understand they were doing something "wrong", i.e. they snuck out. Again, this is normal to want to do.
And what is the best way to tell them?
I'm not sure what you mean by this. "Best" is a request to do something before it happens, but the kids either realized the answer would be "No" and proceeded anyway, or they decided on the spur of the moment (no time to ask.)
...the bemused and relaxed explanation of the other parents might show their son that he can basically do whatever he wants and his parents will be okay with it.
This is one place where "independence" gets very sticky. It's not either/or. There is autonomy (deciding for oneself what actions to take) and there is volition (the degree to which children make decisions about acting based on the morals their parents/society have tried to instill.)
As you said, the kids did nothing "bad" when they acted autonomously. They snuck out to look at the stars. Their "volition" was pretty acceptable.
How should a parent act in these circumstances?
There should be a consequence, but generally, when a high degree of volition is demonstrated, "good" parents should take this into consideration. Higher degrees of volition in adolescent decisions usually strengthen the relationship between parent and child.
...he can basically do whatever he wants...
No. Kids this age (and throughout adolescence) still need a lot of parental involvement and monitoring. Because kids this age make alarmingly bad decisions sometimes.
Without analyzing, parents usually support decisions that demonstrate a high degree of volition, and don't support those that show a low degree.
How I handled it
With grown kids, I can tell you how I handled it when they were going through adolescence. I made it a point not to get angry after the fact, because I wanted to let my kids know that they could tell me anything, and I mean anything. What happened is that I would have a discussion with the child about the action they took. In this case, I would have discussed how important it would have been to me to ask in advance because I would have worried terribly had I accidentally discovered an empty bed. And depending on where they went, I would want them to understand the importance of the risks involved. Since what they did was beyond harmless, it would have ended there, with a "Please ask next time" caveat.
When we were eleven, my best friend and I played hooky one day and took a train into NYC. We wanted to see "hippies". Our plan was to walk to Greenwich Village and find some hippies; that was the extent of it. (Eleven year olds sometimes aren't very deep thinkers.) We didn't even know where The Village was, so all we did was to wander around the city on foot, stopping here and there to shop with babysitting money we'd earned. Both on arriving and departing the city, we went through Grand Central Station, perhaps the busiest transportation hub in the country at the time. When we got back, we wrote each other school absence notes and went home.
I thought I'd gotten away with it until years later I confessed to my mom and she told me she knew all about it from a sticker on a gift I had bought her.
What we did was incredibly thoughtless. But no harm whatsoever came to us. While in retrospect I think my mom could have done better, I think she did pretty well for that time.
Autonomy in Adolescent Development
Towards Conceptual Clarity
Edited by Bart Soenens, Maarten Vansteenkiste, Stijn Van Petegem
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