For all that the show loves throwing these two together, they really never played on this dynamic did they? (As always, you can talk about either of their pre-show lives here in general too!)

Before Manehattan they knew each other about as well as any two ponies (usually) in the same class as each other do (think Diamond Tiara and Pipsqueak before the election). Applejack's big city aspirations may have had some relation to a certain white filly's glowing praise of her winter holiday spent there; and Rarity might have found the idea of one of her classmates making it out of Ponyville ahead of her both inspiring and disappointing - had there been more to the plain-seeming farmfilly than met the eye?

Their most substantial interaction was right after Manehattan, and it wasn't a pleasant one - AJ had little good to say about city life and outright said that no pony with sense would give up Ponyville for that kind of empty dream, while Rarity saw the death of Applejack's ambitions as inevitably foreshadowing the failure of her own unless she could find some way to blame it on a defect of Applejack's character. Both were really talking about themselves more than each other, but it still got pretty ugly, especially when other ponies waded in and started throwing around loosely veiled tribalism. Long story short, neither of them attended each other's cuteceaneras and indeed they pointedly avoided each other for years - until they finally grew up, got over it and started acting like adults.

I guarantee they knew each other, as kids growing up in a small town would, and their lengthy romance rivalry likely started young. Similar in age, they went to school together, their parents probably arranged play dates, but as different as they were, as different as their aspirations and their home lives were, they likely grew to detest any forced interaction. And then the Apples passed away, and there wasn't so much focus on forcing Applejack to make friends anymore.

20. Rarity and Applejack both seem to have grown up in Ponyville. What were their interactions like before the show?
Polite but distant. Rarity would have regularly interacted with AJ in the market, but Applejack wouldn't have reason to visit Rarity's boutique. They respected each other's business acumen but didn't interact much socially.

None.

7039453
A considerable feat given that its' a small town with one school. But I suppose just because a pony's there, doesn't mean you have to pay attention to her.

7017003

Ohhhh, I like this one. Nice, nice.

In my head, they just don't have much common ground. I don't see any reason Applejack would wander around talking about her 'failure' in Manehattan or her lessons learned from that experience, since it's a deeply personal affair. In the same vein, I don't see filly-Rarity already having such a clear vision of her desired path and goal. Foals can be quite naive. In fact, maybe they should be. Innocence of younger years and all that. I adore the image of filly-Rarity thinking that she can be a fashion star... in Ponyville.

Like any 'good' Apple, AJ stayed on the farm most of the time. Always work to be done, isn't there? Family to support, too. She's just... too busy for a lot of things. Over the years, she slowly grew into her role as the new family matriarch. And since Ponyville and the Apples are so closely connected, she became the new support pillar of the community.

She was aware of that seamstress buying that failed old shop over there, but what did it matter to her, really?

In the same vein, Rarity pursued her dreams and with growing age and wisdom, her vision of her desired path became clearer. She tried to get a foothold in Canterlot from early age, but train tickets cost bits and nopony listens to a young filly. She had to prove herself in other ways. And she's nothing if not a shrewd filly with some business sense. If she can get these rural folks around here to appreciate her work, she can make it anywhere! She saw conquering Ponyville in the name of fashion as her first hurdle, her first major challenge. But never in those years on her path did she ever encounter a problem that required the kind of help you call an Apple for.

They just... coexisted with little contact area.

7689095
Hmm... an interesting other take, and one which does explain better than mine why they never talk about their past despite frequent co-focus.