While Magic's current planeswalker vary wildly between themselves in appearance, gender, motives, and personality, one thing is generally consistent: Almost all planeswalkers are portrayed as in their 20s or 30s. Whereas Domri Rade (who is teenaged) and Nicol Bolas and Karn (unfathomably ancient and completely ageless) are notable exceptions, the age barrier for post-Mending planeswalkers has yet to be broken by one group.
Tumblr user solune44 pointed this out in asking Mark Rosewater, "The talk about bearded mages made me realize all the planeswalker look young. I guess it would have less resonance with your overall audience but i would be glad to see an old one once. Say, looking like a 60-70 years old man/woman. Do you think it could happen."
Mark's answer was:
Yes, I feel it will.
And audience responses were enthusiastic - Magic players definitely crave variety from their planeswalkers. Said darththulu, "*One* walker who looks like she is past menopause & “not totally hot” would be huge improvement. Even *Vraska* looks sexy! I’d be thrilled if a block had 5 ‘walkers who looked like Nanny Ogg, Magrat, Granny Weatherwax, Agnes Nitt, & Tiffany Aching," in reference to the notable witches from Terry Pratchett's Discworld series.
Of course, the six billion ton gorilla in the room is the most infamous planeswalker of all time, the architect of the entire Weatherlight Saga, Urza: tivoko wrote "My favorite walker will always be Urza and he’s often portrayed as an older man. I love the way he looks in Mind Over Matter." In fact, we only get to know a young Urza during the Urza's Saga block which is in fact a prequel (well, sort of, since time is complicated on Tolaria) - he's got a quite distinctive wild white mane and goatee in every other set.
Finally, abalidoth had some more concrete suggestions, saying "BADASS OLDER LADY PLANESWALKER YES PLEASE (It’d be a fun mechanical nod if they had abilities that referenced older aspects of Magic, like a lot of the stuff from Time Spiral block.)"