Today was my first day starting (again) with a To-Do apps in order to organize my day.

You used "a" so no "s" at the end

I have noticed that as I get older the differents techniques that I learned when I was younger have a more powerful impact on me.

My theory is that younger people hasve other interests, they prefer hanging out with friends thanover developing themselves .

I mean you can urge them to study more or be a better person, but the chemichals in youtheir brains are crazy and don't let you tothem focus ion important things.

I'm not who the "subject" of this sentence it, meaning who we are talking about. It seems like you should be talking about kids in general, so i used they.

Sometimes I'd liked to behave been more mature back then but well, that's the beauty of life.

Había estado i believe is what you're saying so have been

Today was my first day starting (again) withto use a To-Do apps in order to organize my day.

I have noticed that as I get older, the differents techniques that I learned when I was younger have a more powerful impact on me.

For instance: Sexample, studying techniques, Ttime management etc.

My theory is that younger people hasve other interests, they prefer hanging out with their friends than developing themselves .

But that's not their fault, I think that's because their Bbrain is still developing.

I mean you can get the urge to study more or to be a better person, but the chemichals in your brain are crazy and don't let you to focus ion important things.

Sometimes I'd liked to be think that I'd have liked to have been more mature back then, but well, that's the beauty of life.