Data was 26 years old in the first season, 11 years older than Wesley, (1.733 times as old) and yet was not so humanly socially advanced as Wesley.
Obviously the writers wrote stories and scenes showing Data becoming more Human-like during his seven years during TNG, and not during his previous 26 years of activity, because scenes set before the first episode would never be seen. But we need to explain why Data suddenly made a lot more progress in his goal of understanding emotions - especially Human emotions - so comparatively late in his experience.
Theory one:
Data did not remember anything about his experiences on Omicron Theta before he was deactivated there. But it is possible that he had subconscious memories of the (mostly or all Human) colonists there and his Human creator Doctor Soong, which may have imprinted in his positronic brain a preference for dealing with Humans.
If the crew of the starship Tripoli (named after Earth cities) who found Data and reactivated him were mostly or all Human that might have also imprinted Data with an interest in Humans. Cronologia de Jornada nas Estrelas: A História do Futuro says that Data entered Starfleet Academy 3 years after being reactivated and graduated 7 years after being reactivated.
Data's only mentioned Starfleet assignment in the 19 years between graduation and "Encounter at Farpoint" was being on the starship Trieste (named after another Earth city) when it passed through an unstable wormhole.
So one possible theory is that Data spent most of the 26 years before "Encounter at Farpoint" in the company of highly nonhuman beings who were either highly unemotional like Vulcans or had strange emotions compared to Humans, despite being somewhat imprinted on Humans and desiring to spend more time among Humans.
In the three years between being reactivated and entering Starfleet academy Data may have examined by mostly highly nonhuman scientists at the Vulcan Science academy or at an Annex of the Daystrom Institute on a world with a highly nonhuman population.
Data may have spent part or all of his time at Starfleet Academy in a campus on Vulcan or some other highly alien world instead of the campus on Earth, or perhaps attended lectures through telepresence without interacting with Human students.
And Data may have spent most of his 19 years in Starfleet before "Encounter at Farpoint" assigned to ships and starbases with nonhuman crews where he would have little opportunity to study Human psychology. And perhaps Data spent much of his prior Starfleet career on science vessels and research outposts studying strange new worlds and civilizations.
Thus Data may have sought assignment to the Enteprise in large part because its crew was mostly Human.
Theory 2:
It is possible that Data needed to accumulate a critical mass of observations of Human behavior before he began to understand it, and that he reached the critical mass of observations during the course of TNG.
I hope that some combination of my two theories with the other theories presented here will be enough to sufficiently explain why Data suddenly began to understand Humans much better during the course of STTNG.
