Baby zombie horses and baby skeleton horses age and ultimately grow into adults again. This was previously tracked as a bug and fixed in 26.1 Snapshot 7, but it reemerged in 26.1 Snapshot 8.
Incidentally, undead horses use the Age tag instead of the IsBaby tag, which is inconsistent with other undead mobs.
Can confirm as of 26.1-snapshot-10
Baby Zombie Horses and baby Skeleton Horses age.