When should one start to learn a new language?

I’ve always been told that you should not try to learn two or more foreign languages at the same time, even if ( or especially if) those two languages are very close to each other - like Spanish an French for example.
What do you guys think? Is it possible to learn a third language while you still not fluent in your target second language?
Is there a recommendable level of fluency you should acquire first so you can start to learn the other language?

As long as you know what you’re doing and stay disciplined and focused enough, I wouldn’t say it’s detrimental/counter-productive to start learning/overlap/maintain two (or more) languages at the same time.

It all depends on what you want to do, I think. I find that I get obsessed with one language and it is difficult to focus on another at the same time. However, it is beneficial to do so I think. I am still under the spell of my Russian, but when I did spend a few weeks on Portuguese or German or Italian, it was a pleasant break and my Russian did not suffer. But I am determined to get a little better in Russian before focusing on another language. But everyone is different.

Well, in the Netherlands the children in secondary education (high school) learn three languages at the same time: English, German and French. I never found myself mixing up the languages, so I guess it is fine. But if you have limited time to learn, I think I would focus on one at a time.