I heard that old languages didn’t have future tense, and that it developed only in younger languages. Is this true? Why would that happen?

We know that the ancient Greek future tense suffix –s– is derived from an Indo-European desiderative suffix –sy-. In other words the suffix that already in Homeric Greek meant “I will” is derived from a suffix that I originally meant “I want to”, and that in fact independently survived in Ancient Greek with that meaning (khe-sei-ō “I want to take a crap”).

By the way, you know what the future tense I will derived from in English, right? That’s it. It used to mean I want to.

Similarly, the Latin future suffix –b– can be explained through the Proto-Indo-European verb for to be, becoming a suffix and attaching to a verb. I am to go.

Sound familiar?

But the fact that the ancient Greek or Latin future suffix is an innovation does not mean that Indo-European did not have a means for expressing the future. It may have been a different suffix. It may have been a different auxiliary verb. It may have been a circumlocution, like using the adverb soon or tomorrow.

Latin had a future tense. It has disappeared without a trace in Romance languages, which instead use a suffix based on have. Language churns, and we believe language churned in the same way 5000 years ago. The fact that the Romance future suffix is an innovation does not mean Latin did not have a future suffix of its own.