I wonder if most twins really care, or if it’s just kind of a joke that stereotypical older sibling vs young sibling relationships should apply based on one being a few minutes older?
Yeah, there are certain types of birthmarks that are rare , but birthmarks in general are common - although I’m not sure how accurate that 80% is. I’m not sure every baby is checked for tiny birthmarks.
I knew twin sisters in college who did many things together with grace and style. I helped them move once, and with each of them holding up one end of a couch facing each other they could not manage to move in the same direction…
Is this a joke? I don’t really get it.
Was one of them loudly repeating the word “Pivot”?
It does seem silly for it to matter to twins born effectively at the same time. Twins seem to make a big deal about it. Maybe because they like to have a difference to point out. I did meet a pair of women who said they were identical yet one of them was noticeably shorter than the other. That was back before DNA testing though, no idea how they could have known for sure they started as one fertilized egg.
Check out @beowullff’s entry on the Sklar-Crossed Brothers. These guys were uncertain who was born first for a long time.
I went to high school with twin sisters who were not identical, but they were about the same height and weight, and had similar hairstyles, so they looked very much alike from a distance. One day, our history teacher walked in and said, “Jane, I saw you smoking out on the sidewalk this morning.” Jane (her real name) replied, “That couldn’t have been me, because I don’t smoke; you may have seen my twin sister.” The teacher started to say "And since when - " and those of us who knew both of them said, “No, she really does!” The teacher replied, “So, that’s why I see you so often around the school.”
Paint their toe nails different colors.
That takes 10 colors. How do you then tell them apart?
Actually it would be 20 colors doing it your way 
Identical twins don’t quite have identical fingerprints, but they are very similar. That used to be the standard way of telling that twins were identical.
And I have cousins who are (non-identical) triplets, and my aunt really did treat the “firstborn” differently on that account. That particular aunt is, shall we say eccentric, in many ways.
No, no, no! The Limpid Green is on Kelsey’s left middle toe, not Chelsea’s. Chelsea’s left middle toe is Lettuce Green!
…
… I think. Or was that the other way around? … Crap! 
I give up! Let’s flip a coin. … Again.
Fast forward thirteen years, and Kelsey decides to spell her name Celsei, and Chelsea goes with Kjelsee.
That’s the part you thought “is it true?” that it’s rare? I’m skeptical of the claim they’re from being born.
I myself have almost enough Café au lait spots for there to be a concern about neurofibromatosis - guess it’s lucky I don’t have 1 or 2 more.
Yeah, you could get marks from the process of being born, but they’d be as transient as any other bruise or scratch.
I’ve never seen a claim that they are from “being born” - I’ve often seen that they are present “from birth” but that’s a description of timing , not cause.
That’s my understanding. I don’t think anyone in my family has a birthmark. Nothing I could think of anyway.
My twin solution may seem a little drastic at first, but it has a pretty big payoff the twins will thank you for later.
Cut off the left foot of one and the right foot of the other. Replace each with a high quality caster wheel. Just think of all the three legged races they’ll win at family picnics. Win prizes, bet money. Fame and fortune are guaranteed, you’ve set them up for life!
A modest proposal if ever I’ve seen one. Excellent!
I posed this question to a friend who has identical twin girls (about 9 or 10 years old) today. He told me that when they were babies, they kept the armbands with their names that were put on them at the hospital on them for a long time, like six or eight months. They also didn’t dress them exactly the same. Over time, he started noticing each daughter’s distinct features. Today he has no problem to distinguish them. There are both subtle differences in physical features (none he could pinpoint - he would have to see them to recognize them), and differences in personality.
He also opined that until each twin starts answering to a name, it’s not necessarily a big deal if you don’t know which is which. I don’t know if that is a prudent opinion (for example, one child could have a health condition that the other doesn’t and you should be able to readily identify that particular child), but that was a casual statement he came out with.
I suppose there’s a difference between not caring which is which at the moment versus permanently and irrevocably confusing which is which.
Darn near any parent of kids, or any kid w sibs has experienced the parent shouting “Jim, I mean Dan, I mean Susy!, Stop that right now dammit!!” The parent is confused about who is who in the current situation, but is not confused about who is who for the long term. They can tell Suzy from Dan every time once they’re not excited / annoyed.
The situation w identical young twins is fundamentally different.