Building a house at 21 years old .. too young?

Yes, you can still do all that. As long as the monthly burden fits, everything works. If you want to.

But I am NOT talking about the financial aspect again. There are professions where you can't just take a break for half a year. Being involved in extreme sports or experiencing your own limits requires a lot of invested time.
Professional courses, which are spread over several years and completed in distant locations...
I can't cover all possibilities or situations with examples from my imagination either, as I myself have stagnated in only one place. But sometimes I think that if I had "moved more" (had to), I would have had something more in my life, for example, I lack the time of studying or this social component, instead my apprenticeship was instructive but very sheltered. My workplace (public service) also hardly gives me any more opportunities to move around the country. Well then. Not everyone may need that, but do you know how you will think about it later? It is no coincidence that "at 40 and having achieved everything" leads to the male midlife crisis. At least there was this difficult phase in generations who settled down at 20 (professionally and family-wise), but later wanted to enjoy what I call the New Era (internet, booming economy, salary jumps, free sexuality, independence, education, etc).
My opinion is: nowadays everything shifts a bit later, above all we live longer than previous generations, and so one should take advantage of the phases of "physical" possibilities. As I already said...
Of course, that is just my opinion, I also accept other opinions and attitudes. But I am allowed to express my concerns in a discussion as well.

But again, I do NOT mean the financial aspect. There are professions where you simply cannot pause for half a year. Engaging in extreme sports or experiencing your own limits requires a massive time investment.
Professional courses that take place over several years at distant locations...
I can’t cover all possibilities or situations with examples from my imagination because I myself have only been stagnant in one place. But sometimes I think that if I had “moved more” (needed to), I might have had a bit more in my life, for example, I lack the time of studying or that social component. Instead, my apprenticeship was certainly instructive but very sheltered. Also, my workplace (public service) hardly gives me any more opportunities to move around the country. Well then. Not everyone may need that, but do you know how you will feel about it later? It’s no coincidence that the male midlife crisis comes with “having achieved everything by 40.” At least that difficult phase existed in the generations who settled down professionally and familiarly at 20 but later wanted to enjoy what I would call the new era (internet, booming economy, salary leaps, free sexuality, independence, education, etc.).
My opinion is: nowadays everything shifts a bit backward, above all because we live longer than previous generations, and therefore we should make use of the phases of “physical” possibilities. As I already said...
Of course, this is just my opinion, I also accept other opinions and attitudes. But I too may express my concerns in a discussion.

I see it exactly the same way! Absolutely correctly argued.

I see an extremely big problem that the pressure (especially in Germany) is extremely high. Nobody is satisfied anymore, always wants more (especially in the area of job/career). It’s no longer unusual to have two wonderful children at home by the age of 40, a house you own and a secure job. That was the moment when the generation before us leaned back and simply enjoyed it, since they didn’t even have the possibility to think in another direction.

I have to say, I definitely don’t envy us in 2020 for that and personally hope I will be significantly more satisfied than most people are today.

That’s also the reason why relationships no longer last 30 years... There is way too much pressure/influence on people from society from the outside/social media and so on... Really a shame, actually.

Fits quite well. Since we grow up like that from a young age.. Well then, just wait and see what happens to our children in 20 years xD

It somehow seems to me here as if the forum members don’t wish me well. All I hear here is something along the lines of the motto: "You’re too young, you’re too stupid to build a house." Really a shame!

I don’t know if that’s the case; I rather think that some older members want to point you to possible scenarios, which I consider sensible and constructive. If there actually is a jealous person among them, then you’ve already earned that badge.
You have a plan and you are currently implementing it; with that, at least you’re not mainstream, because at that age people often have no plan what they want/should do. I was 30 at my first house and had zero plan... plus a lot of luck but also bad luck... that’s life.
Whether your way is right or wrong no one can judge, because none of us even really know if our own plan is the right one... and what is right or wrong changes quite often even in one’s own life.
described it quite aptly.
You meet world travelers who couldn’t be more petty and a cosmopolitan who prefers to sweep the street in front of his terraced house several times a week; as long as a so-called petty person doesn’t annoy anyone and enjoys his life, he is less petty than the one who complains about him.
In general, I don’t like categorizing, so in my opinion your house building is just as right or wrong as mine with 40 years more life experience. Also, I believe that age alone does not represent a particularly noteworthy achievement.
All the oft-quoted expressions like "blowing off steam," "living it up" suggest to me as if one could follow a plan.
There are billions of reasons why your life will be like this or that, and as long as you do what you decide and take responsibility for yourself, you’re doing it right for yourself. Changes can be just as varied and are not compensated solely by material things; they are anyhow only very limitedly planable. In the often justifiably mentioned issue of divorce, material matters are often calculated, but the really profound damage usually takes place elsewhere, where one cannot foresee or calculate it.
I wouldn’t know whether I should advise my roughly 10 years older children for or against building a house. I would do neither, because I have no idea what would be "better" for their lives.
Still, you should take the critical voices just as seriously and reflect on them, because these usually are based on actual events; backslappers are not always the best advisers. Whether and which advice you then adopt in your situation is another matter.
You decided on it, so have fun with it!!!

You asked the question in this forum to receive feedback. That has nothing to do with generosity. We all don't know you. When you ask a question, you must also expect appropriate feedback, which you alone have to process and reflect on. Don't underestimate the changes you go through between 20 and mid-30s, but if it's your dream, then just do it. You learn from mistakes after all. This is not meant to imply that it is a mistake. Personally, I think you are too young. You should start building assets at a young age. A self-used house does not belong to that. Then one more note: The first house is built for enemies. The second for friends and the third for yourself.

Everyone has their own experiences, which of course are not universally applicable. The thing about getting together young was my experience in my environment. I was also already in a relationship at 24. At that time, I consciously decided against a house.

So now, better show the house because that's what this is all about.

Many people only finish their studies at the age of 27 (well... I wonder if that's really so great and if the person has truly lived it up). Many are still getting drunk every day at 30 and thinking about where the next party is happening. I see that in my environment... 90% simply wouldn’t be mature enough for a house. There are couples who have been together for 8 years, are in their early 30s, and their future plans, as well as what they have experienced together so far, are not even a fraction of what we have already experienced in our early 20s.

Exactly for that reason, it makes so little sense to compare yourself to others. You never really know what's going on under the surface with others. I also know couples who have been together for 8-10 years and get married to fix their problems. Hopefully, a child won't come along soon to fix the problems that the marriage added... Well, not my issue. I also know plenty of people in their 30s to 40s for whom "partying" is the holy grail. Who knows what reality they’re fleeing into the booze? I don’t, I’m just grateful to be spared that.