You can't use a golden dandelion on a baby zombie horse

You can, but it has to be tamed using this command:

/summon minecraft:zombie_horse ~ ~ ~ {Age:-1200,Tame:1}

However, judging by this Bedrock Preview changelog, under “Mobs”:

https://feedback.minecraft.net/hc/en-us/articles/41627058435213-Minecraft-Beta-Preview-26-0-23

“Fixed a bug where baby Zombie Horses would shrink in size when they grew into adult Zombie Horses

• They cannot now grow into adult Zombie Horses at all, matching Java Edition”

It seems baby zombie horses (and by extension, baby skeleton horses) aren’t meant to age at all.

Almost definitely working as intended, undead mounts are not supposed to grow up.

Working as intended now, as 26.1 Snapshot 6 added the undead horses in the cannot_be_aged_locked