Yeah, I've got a few more questions here.
Can youkai die of age? I heard they did. If so, what is their average life span? And do only certain types die of age? And do they actually get to look older? I can imagine vampires like Flandre and Remilia to be unable to die of age and stay young, and Yukari probably gap haxes her youth constantly, since she's apparently way older than a youkai should be and still looks like a bombshell. Also, do Shrine Maidens and Demi-Gods get older than average humans?
Do youkai have a predetermined amount of power and potential, or can they increase both with training or artifacts? For example, was Yuuka (genetically) destined to become so strong, or did she achieve it through sheer work and effort? Can Wriggle for example ever hope to be as strong as Yuuka?
Does Gensokyo grow, or is it stuck at it's current size? I can imagine the ever growing population of youkai and humans are going to need more room eventually.
You have many many questions in a single post my friend.
I'll go one at a time.
Ans. to First Question.Almost everyone can die of old age, the only ones that can't are people like Kaguya and Mokou as well as the gods who keep on living until they're forgotten.
The average lifespan is hard to say, but if you look at the original stories of some youkai, like Ran the nine tailed fox, its said that it takes 1000 years to reach nine tails.
Kogasa's origins thought to be the Tsukumogami take 99 years to become a tsukumogami.
...So for most youkai, you're looking at a broad range so it would be meaningless to get an "average" of anything.
Although Keine is half youkai, she's abit special. According to her she only lives "a bit longer" than most humans. So her lifespan thought to be quite similar to humans.
Many youkai can look older if they want since most of them change in to human looks since its only "more convenient", as Orin says. In cases like Ran and Chen who are youkais originally known for using techniques on deception especially.
So there's nothing against a youkai who is not originally in human form to not look the way they want to look.
... Well looks is really a difficult topic to talk about when the official reference to looks that we have is so little...
Oh, and shrine maidens still get older, they are human after all. Also, I guess you're referring to Sanae when you say demigod, but in her case, she's a human in the first place, so unless she's worshiped after death or something and turns in to a real god, her life expectancy is no different than an average human.
The second:
It is said that youkais mature in strength and knowledge as they get older. So it's just that some youkais in Gensoukyou have lived so long that they've reached an almost godly level. (It's like trying to compare different characters with different levels.) So youkai who are considered to be of lower power just haven't lived the years. (Notice that many of the "strong" beings in Gensoukyou have lived countless years. Ex.Yukari, Ran, Yuuka,...etc.)
Since the only actual blood relation that we see in Touhou is with Suwako and Sanae (and possibly Alice and Shinki), we can't make any assumptions on genetic stuff.
So if youkai like Wriggle was to become stronger than Yuuka, it would be after Yuuka died and Wriggle lives on for a longer time than Yuuka was alive. (That is, not factoring in their abilities, if its abilites alone, Wriggle is already beating Yuuka...although Wriggle is still not smart enough to notice the extent of her abilities.)
As for the third.
Gensoukyou probably does not grow, think of it sort of like a spacious city/town, its population can grow, but its far from reaching the limit.
... Of course Yukari can always fiddle around with the borders...
