Is it possible to learn a language solely from software and textbooks or would you need to interact and learn from a real person?

I don't see why it wouldn't be especially since you have access to tons of ressources even to practice your pronunciation or having access to particular slang of a language. It'd be 10x times harder, and longer though surely.

Thanks to the internet you can listen to and speak with real people though. If you really apply yourself, you can learn X language (provided it's a language with a decent amount of internet presence though) by watching youtube and skyping with someone, besides the obvious self-study

It's easy as a child, hard as a teen and nearly impossible as an adult. I wish I had learned some other language when I was young.

For language-learning I believe that's considered an 'adult' age according to research though

I think it's possible next year I want to try a cute Czech course they organize her in Milan at the Czech Culture Center

I only use textbooks for basic grammar rules but other than that practicing it online with native speakers is always better and really helps to you learn it faster.

My language learning is slower and worse in general than, for example, 4 years ago when I started Spanish. Mandarin Chinese is a bit of a challenge now, but I'll do my best, 'cause it's not impossible.

Scientifically speaking, yes it is. Your brain gets less malleable and adaptable as you grow older, so it's far better to learn languages from a young age. Soon after you speak your first language is often cited as the best time to be taught another one. It's possible to learn a new language when you are older, but it takes a lot more work.