I think "Sebastian" is pretty young, and hasn't really been around humans for long periods of time before now. And our Ciel is definitely the first child he's interacted with.
Sebastian is a young demon?
That's pretty much my thoughts on it. For a demon, he might be "young" and somewhat inexperienced, despite any humble bragging or outright boasting.
At some point, the demon gives up "messily sampling" souls and starts accepting contracts. But the only hints we have about his previous contracts suggest there might not have been many. In fact, we only have hints about two previous contracts: one in the second half of the 18th century in Austria and maybe one in the later 15th century in Italy. Those assessments are based on things he says about the Viennese Waltz and the outfits he changes into when our earl seems shocked by his first appearance as a human.
If he slept between those contracts, that means a lot of time spent in his own realm, away from human contact.
I wouldn't say our earl is the first child he's ever interacted with, but he's definitely the only child contractor he's ever had, and he says so. He admits he's not used to caring for a human who can do so little for themselves, and he's not used to masters who expect the demon to cook from scratch, appear to do things the human way, or generally "act human". And even though Sebastian suggests a human lifetime isn't a long amount of time for him to wait for his "meal", Othello suggests that most contracts don't last long at all, because most humans have very simple wishes when they make contracts.
The demon learns knowledge and skills from contracts, and judging by the things he didn't have skills with or knowledge about (like choosing dish patterns [according to the Character Guide], making tea, cooking, bathing a person, water temperatures that harm humans, particular illnesses like asthma, or even how to get the best work out of the other servants), he's never had contracts before where those skills or that knowledge was required.
And I think demons learn emotions from devouring souls, especially ones from contracts. Seems like the souls he's devoured in the past were somewhat lacking in emotional range. Perhaps those previous masters had experienced no true sense of love or loss. Not like his current young master.... They were probably truly more selfish individuals.
