13 year-old Princess Yvonne of Sayn-Wittgenstein-Sayn and her brother, Prince Alexander, enjoying some royal luxuries, Germany, 1955

Were these plastic cups already standard back then?

I think those might actually be frosted glass cups.

Here is the best copy I could find, and the flared edges do not look like plastic cups - also the shiny inside and matte outside looks more like glass frosted on the outside.

PP was not in commercial use until 1957 (not sure when it was first used for cups), PET disposable cups weren't a thing until the 70s. Disposable cups in the 50s would have been wax paper.

Looks like someone wanted them to pose as such to make a funny photo

I'll take "What's classy when you're rich but not when you're poor?" for $100 Alex

Is it that classy?
- Given the monarchy is the top class in a class system then yes: by definition.

I'm sure they grew up to be normal, well-adjusted and down to earth kids.

This photo became the notorious cover image of Dinosaur Jr.'s Green Mind, as us older folks well remember.

Is that a cigarette in his hands? He was 12 years old in 1955.

I was smoking at 12 in France in 1999, I told tobacconists I was buying them for my mom. Pulled the same thing off in the states when I moved there 2 years later. And I was far from alone. These are far more recent (terrible, obviously) habits than you might think.
- Started smoking at 12 too, also in the 90s. Quit in high-school.
Well, the princess is downing a bottle of fortified wine, so it would seem that's just par for the course.
- When I was 14, my parents ordered a bottle of wine in a restaurant and asked, if I wanted a glass. Nobody thought twice about it.
In 1955, basically everyone smoked. Like 65% of Europe were smokers, and I do mean 65% of living humans of all ages.
My dad was smoking at 6.

Rural Canada, 75 years ago. Still alive.
- Cancer is nothing you can count on. For the bad and for the good.
  Hope your dad stays healthy.

Amateurs. I started my cocaine addiction at 5 years old when the Fleetwood Mac album Rumours came out.

Edit: again I need to learn doing correct grammar in English.

again I need to learn doing correct grammar in English.

It's pretty decent for someone nearing a half century coke habit.
- I am getting all emotional, "snif".

13 going on 30

How can you still be prince from a county that has ceased to exist for centuries?

I read this as a non-rhetorical question, in a Nigerian accent
- Me too, especially due to the use of the word 'ceased'. Reminded me of this video where a reverse scammer convinced the Nigerian scammers to record Monty python's dead parrot sketch, as he combines then he could get them a BBC grant.
  
  https://youtu.be/awtAKa22AVk

I come from lower class but tbh reminds me of my early teens too. Booze and smokes.

Is she... Married?

I think even Euronobility stopped the practice of marrying their kids off THAT young a few centuries before.
- Why the ring on the finger then?

Those sure were the days

Source? But their hands look like poor AI garbage...