When it comes to life financial investment I don't judge it by how early you have started making an investment plan, I only judge it by how far you going to, end up finishing it. For example, those that invested in Bitcoin in its early stage and those that invested when it was a bit costly. You will get to find out that many sold off their bitcoin holding at a lesser price at the early stage when the price of one bitcoin is around $5k, and some even bought bitcoin when it was around $15k and they are still holding till now.
The paradox of an early financial investment plan is how well you end up finishing it whether early or late
Honestly, those people who think that if they are married with kids and can't try new things do not realize how many hours in a day we have. I have seen so many people who just hours and hours on useless stuff that if you are planned and organized then you have plenty of hours left. I used to work in a different business industry, I worked there for 4 years, and in 2 years of that period I worked every single free time I had, while married (albeit no kids) and still spent time with my wife, all thanks to being planned and organized. Still doing the same thing, my work takes a good chunk of my time, but I don't waste the rest on just relaxing all the time, I try to develop new skills and use them for fun instead of work now.
Honestly, the biggest mistake someone could make and it will affect him or her all through their life is going into marriage when not financially ready for it. In this case, it is better to start late and be financially buoyant than to start early when not financially ready for marriage. This will determine your future and the future of your kids
I had been in the Bitcoin scene since 2014 but stuck around only for a short while before re-entering the scene a few years later. I made plenty of mistakes that, at least now, I'm not repeating, and I've stopped looking back and thinking about what I could have done differently. As you've already mentioned, there's a decent chance that the majority of those who were here in the early stages would have sold during the first bull markets. Life is full of lessons. There are mistakes that I've made in my career, like switching jobs and ending up in a worse one, but regrets lead nowhere.
Now to address the marriage topic you mentioned: getting married when you're not financially ready is neither the largest nor the most complicated issue. A young couple living together with two jobs can live quite comfortably with an average job and sharing expenses in half; the issue comes when kids are around. However, the worst mistake you can make, both on a financial and personal level, is marrying the wrong person.