Early Childhood
Arthur was born in early 1859, and touted as an easy success for having a penis. When his father died tragically scarcely a year later (1860,) this seemed even more lucky - Andromache would only have one child, but he was a boy.
By the time he was four (1862) Arthur had solidified his reputation as a pain in the ass for the nanny. Partially out of desperation, his mother stuck him on a toy broomstick - and the rest was history. Demonstrating magic by zipping around on the thing into ridiculous situations all the damn time, Quidditch was hands-down Arthur's first love. The household staff, out of necessity, becomes good at healing magic.
Arthur's childhood is framed by his very close relationship with his mother, who spent more time with him than most rich women due to his lack of a father. He adored her utterly, and was happy to be taken to events with her Lestrange relatives.
HOGWARTS
Once he went to Hogwarts in 1870 with a pet toad in toe, Art surprised absolutely no one by being sorted into Gryffindor. He made fast friends with the likes of Ben Crouch and Fitz Prewett, and was a sociable if not particularly dedicated student. He took some easy-seeming electives (COMC, Divination) and finally joined the Gryffindor Quidditch team in 1873. Arthur would serve as Chaser for the rest of his Hogwarts years, and was more devoted to Quidditch than his studies - although he did pass enough of his OWLs (1875) to stay in school, obviously!
It was really after his OWLs that Arthur's tendency towards trouble starts to become apparent. That's the summer that he starts actually partying. An excursion to the casino with some school friends starts an itch, and Arthur becomes rapidly convinced that with a few good investments, he could double the Pettigrew fortune. He's young enough that no one really listens to him yet, and he does not have access to the fortune yet. If anything, his mother is more concerned about his fondness for drink. This is also the summer that Arthur becomes really interested in girls - (and the occasional boy, later) - and it's the next year, in 1876, that he loses his virginity to a girl in the year above him.
Art decides in his seventh year that really he wants to play professional Quidditch. Given his coming inheritance no one pays it much mind at the time. He is drafted to the Kenmare Kestrels as a second-string chaser, (boo, he doesn't get to play with Ben anymore!), sits his NEWTs to mediocre results, and graduates. His father's will stipulates that Arthur gets the money and everything when he finishes with N.E.W.T.s, and this is when the money kicks in - and very rapidly becomes a problem.
THE FUCK-UP YEARS
It all starts in 1877, although it really isn't for another year that people catch on.
Attending Quidditch parties and desperate to impress his various crushes, Arthur plays high-stakes cards with other wealthy players and sponsors. Usually, he loses. This wouldn't be a problem on its own except that he still feels that itch, and towards the end of the year, he also starts making high-stakes bets on Quidditch games and Hippogriff races. Problem.
But what really gets attention is in early 1878 when Art makes a bet against some goblins. He loses, because no one ever wins against goblins, and goes underground in the off-season while they look for him. Although Arthur thought this would make his debts go away, it doesn't, and when he emerges the goblins 1) break his nose and 2) sue him for the Pettigrew property. So they lose the big house and move into a smaller one in London, and maybe this would even be okay, except Arthur keeps making bets. It's as if he can't stop, even though he wants to.
So they lose the smaller house, and they rent, and then the next thing Arthur knows is that by January of 1879 there's no money in the accounts, he's dropped a class, and none of his relatives will talk to him - other than his mother, who now has to work. Andromache stands by her son, for whatever reason.
But he's left with almost nothing and is a perpetual disappointment who drinks too much and tries weird drugs and does low-stakes gambling, and the one perk is that in 1879 he's promoted to first-string. Which, well, bully for him.
QUIDDITCH'S MOST FAMOUS?
In 1880 Arthur's mother moves out to serve as a governess to an upper-class family, leaving Arthur to live with a rotating cast of Quidditch friends/human disasters/low-key opium addicts/other gamblers. As you do. His residence rotates between London and Hogsmeade depending on the season and when he infuriates whoever he's living with. He probably would have been happy to continue #disasterlife for several more years, but in 1883 Arthur tries out for the British National Team.
He not only makes it first-string, he's the captain. Although they don't win, the pay and the spotlight at least inspire Arthur to try a little harder to be an adult. He rents a house in Bartonburg. He mends things, somewhat, with his mother. He switches teams to captain the Hogsmeade Howlers when they're founded. In early 1884, his gambling slows to a trickle and - stops.
Somehow stuck in Irvingly during the plague, Arthur contracts it (1884) and is visited by a teammate, Miss Desdemona Collins-Potter. He finds himself quite struck by her even once he recovers, and would likely have acted on it much sooner had he not been arrested for a Creothceann death at the end of the year alongside Fitzroy and Freddie Prewett. Stuck in Azkaban, it's protest by teenage girls and the lack of a desire to prosecute Prewetts that gets Arthur released on no charges two months after his arrest (Oct. 84.) #ha. He is left quite shaken by the stay.
In 1885 after a series of romantic overtures, Arthur begins courting Dezzie and proposes to her on broomstick after a particularly dramatic Howlers win. She says yes (Merlin help her) and the two marry in September. This turns out to be, more or less, the last straw for his mother - she is much less inclined to talk to him after he marries a halfblood.
Art agrees to use contraceptive measures with Dezzie so she can stay with Quidditch. In 1887, he meddles a great deal in Ben's love life, and is very, very bored when his friend is shipped off to Canada. A Witch Weekly article speculating that he's impotent and heavy drinking when he visits Ben force Art to admit to himself: he wants kids. He really wants kids.
1888 brings his wish, although not on purpose; Desdemona announces that she's pregnant, but she's not ecstatic about having to retire, and the Pettigrews have a blowout fight about the pregnancy. She gives birth to Gwenog Pettigrew in the early hours of 1889, and Art is immediately smitten with his daughter.
1890 brings a couple developments: Ben's elopement with a woman Arthur hates, and the Quidditch World Cup's return. Art tries out for the team and makes first string, but isn't captain this year, and feels distinctly awkward about it.
Things start to go south during the Quidditch final. Desdemona miscarries, and when she slides into a depression Art starts to slide back into his old habits - drinking heavily to avoid thinking too much about it or, worse, talking about it. The Pettigrew marriage is strained at best, and in January Art finds himself back where he was ten years ago: going to casinos and justifying it to himself.
It hasn't gotten too bad, yet, but he hasn't verbalized it, either.