IIRC, one Rioter stated that Xayah was either 23 or 24, while Rakan was either 27 or 28. Not sure if I'm 100% right, but I think it was mentioned or confirmed in one of their Q&A threads on reddit. Or rather, that might be what their ages would equate to in human years, given the fact that they're vastaya and that many vastaya tribes/individuals/etc. tend to have longer lifespans than humans and all.
So that's a 3-5 year age gap, give or take (if we're going by human years, I guess.)
But IMO, Rakan never looked "too old" to me in his splash art & his concept art, or with his in-game model. IMO he definitely looks like someone in their late or mid-20's.
https://gamepedia.cursecdn.com/lolesports_gamepedia_en/3/33/Rakan_concept_1.jpg?version=24a96d8865216965defb09a398af70d2
https://gamepedia.cursecdn.com/lolesports_gamepedia_en/f/f4/Rakan_concept_19.jpg?version=5756253cacc1f68fc8033b85d7c9267d
https://gamepedia.cursecdn.com/lolesports_gamepedia_en/3/3d/Rakan_concept_2.jpg?version=eccdf5dd63a3693a5cb4e5d9c015b173
Though I have to agree that he did look pretty "off" in their release-cinematic, but so did Xayah in a pudgy, baby-faced, uncanny-valley effect sort of way.
https://i.ytimg.com/vi/8sxR4PEamUU/maxresdefault.jpg
My guess is that the cinematic was either outsourced to a 3rd-party studio, or Riot's in-house cinematic team tried to perfectly replicate their 2D designs into 3D transitions without taking any... uh, "artistic liberties" into consideration, and this resulted in both of their 3D renditions looking "weird" and "off."