IIRC the offical lore is that Umamusumes actually are kind of like elves, they don't really age in terms of looks for a while at least in their higher school years. Hence why "Been Student Council President forever" Symboli Rudolph looks the same after many years.
Though apparently they eventually do age enough to look older, as evidenced by Light Hello, who is a Tracen alumni.
Just you wait! I'll also change my clothes!Trainer-san... Now that a new scenario's here...Are you... going to throw me away...?girl....?Rather than an older lady like me...N-no, that's wrong!What's wrong about it!Young...You even forced her to wear such a thing...!Like I said, that is seriously a misunderstanding!I knew it... you prefer younger girls, don't you...There's always a young girl by your side...!You prefer vivacious younger school girls, don't you?!Uniforms! It's the uniform, isn't it?!Eh...? Ah, no, that's not what I...Eh...?
