What is the earliest thinking beings could have developed in the universe

Not a bad question, but unfortunately one to which we have absolutely no answer. The trouble is that there are two essential questions to which we don't have an answer. What are the preconditions for life, and, given those preconditions, how likely is life to arise?

If we knew the answers to those questions, we could have a go at estimating, at least for a given volume of space, when life might possibly have started first, and from then we could perhaps (and I do stress perhaps) come to some conclusions about when intelligence might arise. However, since we really don't know the answers to either question, we really haven't a clue.

There is one tiny clue, and it suggests that intelligent life may not have evolved early or often. If we assume that life started here, rather than being 'imported' ob a meteorite or comet or whatever, then clearly the preconditions for life were met here at some time in the past. The odd thing, and if you think about it it is really very odd indeed, is that as far as we know, life only started here once. In other words, all living things on earth have a common ancestor. That isn't really what you might expect. Once the reconditions for life were met, you would think that life might pop up independently all over the place on multiple occasions. As far as we know, that just wasn't the case. That might suggest that life is actually very improbable, in which case other life-forms in other star systems or galaxies may not have a start on us after all. If there is life anywhere in the universe at all, of course.

That would entirely depend on the rate of evolution on their planet.
Ours has been quite fast, but repeatedly set back by catastrophes and mass extinctions.
The dinosaurs would have become intelligent if they hadn't been wiped out.

On the sixth day.