For a series about an eternal, repetitive conflict between good and evil, The Legend of Zelda has a lot of variety, except for in the ages of its main cast. Link and Zelda specifically are always rather young, but making a game in which they're older could lead to unexplored character dynamics. The upcoming sequel to Breath of the Wild appears set to expand on the relationship between Link and Zelda, but other new games could potentially be better with a princess and a hero who have known each other for a few decades.
In the entire series, Link is uniformly a child or young man, suddenly tasked with stopping some ancient or powerful evil. Breath of the Wild's Link is technically old because he was in stasis for a century, but he still appears youthful. Princess Zelda is in the same situation, with the games hardly ever discussing her age exactly, but hers usually appears to be roughly equivalent to Link's.
Additionally, these ages usually put a cap on how long the two have known each other. The only relationship between Link and Zelda that doesn't feel serendipitous is the one depicted in Skyward Sword, where the two are childhood friends. Skyward Sword is a good example of how narratively powerful an established relationship between the two can be. Link isn't just pulled into saving the world because he's destined to do it, but because he dearly wants to save his best friend. This could be explored further in a direct Skyward Sword sequel, but unless there's a significant time jump, both Link and Zelda would remain in their late teens or early twenties.
BOTW Introduced The Perfect Dynamic For An Older Link & Zelda
Link is a character purposefully made mostly silent so that players can easily insert themselves in his place, a narrative device compounded by Link usually going from an unknown commoner to Hyrule's savior. Breath of the Wild subverts this well by giving Link a past with the game's major characters, but wiping his memories so that he and the player can discover that past together. Link doesn't always have to be a blank slate, though, and that won't be the case with BOTW 2 - nor was it with any of The Legend of Zelda's other direct sequels.
Pre-Calamity Link serves as Princess Zelda's personal bodyguard, an occupation easily grafted onto an older Link. Although Hyrule may not have been threatened when he was younger, an older incarnation of Link still would've been born in possession of the Hero's spirit, a trait that gets his capabilities noticed at a young age, fast-tracking him into the royal family's service. When Ganon does finally come to conquer Hyrule, a deep-rooted relationship with Zelda and a sense of duty to the kingdom make for more interesting motivations than platitudes about the Hero's destiny.
Giving players control of a Link centrally important to the kingdom of Hyrule offers a variety of interesting narrative opportunities. Rumblings of dark times ahead could truly show players the power of Ganon by having the game's early half lead them through a prosperous Hyrule before the King of Thieves' conquest. Ganon's return is often prophesied, and giving Link a chance to investigate could be a fascinating change of pace for the series. The Legend of Zelda rarely provides deep characterization for Link and Zelda, but increasing both of their ages is a simple way of introducing new dynamics and deeper character development to the series.