Languages are not static entities. They constantly change.

Some, like Latin, are spoken by different groups that end up changing so much that what once was a single language (in that speakers can all understand each other) into multiple languages (speakers of each variety can no longer understand the others). So sort of counter-intuitively, Latin is not actually dead. No one ever stopped speaking it. Instead, different groups just got less and less similar, and there's no discrete point where we can say, for instance, Latin stopped and French began.

So asking what the oldest language is is actually a kind of nonsensical question. All languages change. We don't have a very good quantitative handle on the rate of change (as it is a number of heterogeneous processes), but English today is not the same thing as English 250 years ago, or 500 or 1,000 years ago.